Sigma

"No one can match my power. Hahaha!"

- Sigma's victory

Sigma (シグマ), formerly referred to as "Commander Sigma", is the primary antagonist of the Mega Man X series. He serves as the final boss in all mainline games in the series except Mega Man X8 where he is the penultimate boss in Normal and Hard mode. Once the noble leader of the Maverick Hunters, Sigma fell into insanity and turned Maverick himself, becoming the leader of all haywire Reploids and waging war on humanity.

Overview
Sigma was considered Dr. Cain's masterpiece and the finest of the Reploids, and was the first leader of the Maverick Hunters; peace-keeping Reploids who defend humans against their renegade counterparts known as Mavericks. Although he was once honorable, Sigma unexpectedly goes Maverick at the beginning of Mega Man X and rebels against humankind. He defects to the Mavericks and assumes the role of their leader. Under Sigma's command, the Mavericks grow into a legion dedicated to the extermination of the human race. Sigma instigates many of the destructive Maverick Wars, with the Maverick Hunters existing to crush Sigma and his minions.

Although he has been destroyed countless times, Sigma's programming always survives and builds himself a new form, ready to menace the world once again. It is implied that he grows more insidious and vengeful with each transformation, with each successive plan becoming more and more desperate and aggressive. His primary enemies are X, Zero, and more recently, Axl.

Appearance
Although Sigma has taken many drastically diverse forms throughout the X series, his humanoid design remains fairly consistent. He is arguably more anthropomorphic in appearance than even X or Zero, possessing a bald human head with a cleft chin. His eyes are bright blue or white, possessing no irises or pupils, and with a scar torn over each eye (sometimes the scars seem to be merely an aesthetic, such as in Mega Man X8 where they are replaced by thin lines that run down his entire face). He always has a very tall and bulky frame that evokes the look of a muscular man, and his color schemes often include green and/or purple.

Personality
Sigma was originally an honorable and selfless leader of the Maverick Hunters, but after his corruption, he became a power-hungry tyrant, although even then, he also genuinely believed that he was doing what was in the best interests of the Reploids. He is proud and typically looks down on others, not hesitating to kill the weak and laughing maniacally at their expense. His previous "protection through aggression" philosophy has mutated following his infection into a huge Darwinism complex, believing that anything that is too weak to progress should be annihilated, a la natural selection. Following his defeat, his further descent into madness transcended death, trying to hold onto any grip in the living world, in any form possible. Amidst his madness, Sigma is a calculating and intelligent individual, and on more than one occasion, turned allies of X and Zero against them, and instigated many of the most destructive events of the Maverick Wars, including the Earth Crisis. Sigma is obsessed with Reploid evolution and superiority and believes humanity has run its course. Despite his brutal and tyrannical disposition after going Maverick, he still had some reservations, as he noticeably was disgusted with Berkana and the lengths she was willing to go to satisfy her megalomania, to the extent that he even went as far as to thank X for taking her out.

Sigma was indifferent to X at the time of his first attack, but in their repeated battles he recognized X as a strong Reploid and became obsessed with either possessing or destroying him. That said, he admits in X8 that X's dedication to justice also in part inspired him "to fix" the imperfections of the world. Also in spite of being enemies, Sigma showed interest in Zero, too, repeatedly researching about his past and encouraging the awakening of his original nature, and crediting his infection with Zero's virus as the cause of him becoming a Maverick.

In the remake Maverick Hunter X, however, he was shown to have taken a keen interest in X's uniqueness even before his rebellion, wanting to acquire it to unlock Reploids' potential so he could put it "to the ultimate test" (presumably by eradicating all humans) and lead Reploids into a new age. In addition, the Vile Mode and to a certain extent the main game imply he feels little more than annoyance at the destruction of the Mavericks under his command, thus making his claims about his actions being in the best interests of all Reploids seem at best unqualified by empathy and at worst self-serving; he also showed absolutely no sadness at X destroying Velguarder and if anything congratulated him. His being unqualified by empathy was further hinted at in Day of Sigma, when Dr. Cain, upon Sigma debriefing him of the events of the Maverick attack that day and specifically of how X was handling being a Hunter that Sigma "didn't have that problem at all", referring to X's tendency to hesitate when fellow allies are at risk. However, at the beginning of Vile Mode, when Sigma gives Vile the opportunity to leave regardless of whether he'll aid Sigma in his rebellion to defeat X or not, he specifically states that he "need[s] a Reploid who can go Maverick of his own accord" in his plan to defeat X, implying the possibility that Sigma's becoming Maverick was not entirely of his own choosing but due to external measures like in the original story-line.

Abilities
As the former leader of the Maverick Hunters, Sigma is a capable military leader, described as possessing overwhelming combat power and strategic intelligence. When he turned Maverick, Sigma remains a charismatic leader, and is revived on many occasions by Reploid and Maverick admirers alike. Due to the virus in his circuits, he can survive seemingly anything and always return for more. This survivability was even referenced by Sigma just before finishing off Gate, when he claimed that he didn't need Gate's help to be revived, nor for that matter was he actually dead. He is a very powerful Reploid and is fond of incorporating himself into battle suits which are vastly larger and more powerful than his "regular" form. Sigma is also a skilled manipulator, managing to turn various Reploids, Maverick Hunters, and Mavericks to his side. In fact, there were few instances in the entire series where Sigma wasn't pulling the strings. Sigma himself is one of the strongest Reploids ever created, due to his original role as leader of the Maverick Hunters. His personal energy has been stated to be too powerful to even get an accurate reading. In addition, Sigma's combat skills and analytical abilities were extensive enough that he was able to wipe out several Reploids in an efficient manner, as Zero noted when investigating the crime scenes on the days leading to Sigma's revolt. Likewise, Sigma was also heavily implied in The Day of Σ to be skilled at computer hacking, as he managed to delete any traces of him being behind the first attack after silencing the Reploids who were involved in the first hack, and later it was shown that the Maverick mechaniloids in the second attack were being hacked via the missile launch area, with Sigma being the only one in the vicinity.

Sigma is capable of moving at great speed, often able to move faster than the hero can follow, vanishing from sight. Many of his bodies are also capable of teleportation. Sigma seems to favor saber and beam sword weaponry, often integrating powerful sword combos with his blinding speed or teleportation. On several occasions, he projects energy barriers around himself to block incoming attacks. In nearly every appearance, Sigma is capable of discharging enormous destructive blasts or waves of power capable of covering entire battle areas. Sigma's physical strength is considerable. In his first body, during a conflict with the original Maverick Zero, he was able to effortlessly break through Zero's defenses with a round kick, as well as easily toss Zero through the ceiling with a single hand. He has also demonstrated the ability to scale a skyscraper with a single leap.

Sometime prior to Sigma's first defeat at the hands of X, his Reploid soul became intermingled with the dangerous Maverick Virus and formed the Sigma Virus. This allowed Sigma's spirit to remain after his body was destroyed, granting him immortality and the ability to return after the destruction of each subsequent body. The Sigma Virus possesses the ability to travel through any material or energy and can manipulate both Reploid and organic hosts, even on a planetary scale. Its intangibility allows it to ignore physical assaults and energy attacks, except for those tuned with a Virus Buster to target the Sigma Virus's unique matrix. No solution for the Virus remained ultimately effective during 21XX-22XX, as Sigma demonstrated the ability to adapt to any countermeasure. As the cause of death and suffering for countless innocent lives, Sigma is one of the most dangerous villains in the classic Mega Man universe.

Creation and Origin
Sigma was created by Dr. Cain as his magnum opus, considered the most advanced Reploid of his time. He was put in command of the Maverick Hunters after their formation, putting Sigma in charge of worldwide peacekeeping efforts. He specifically commanded the 17th Elite Unit, leading them to victory many times over the years. One day, the Maverick Hunters received reports of an overwhelmingly powerful "Red Maverick" that had destroyed an entire unit of Hunters. While investigating, Sigma and his unit tracked the Maverick down to an abandoned laboratory; Sigma decided to confront it alone, so as to not endanger his colleagues. It is here that Sigma encountered Zero, the last product of Dr. Wily. Zero, having suffered a programming flaw from his creation, was uncontrollable, unleashing wanton destruction wherever he went. Sigma was caught off-guard by how powerful Zero was, and suffered heavy damage at his hands. Fortunately, Sigma was saved when a 'W' appeared on Zero's forehead lens, causing him great pain and allowing Sigma to deal the finishing blow by punching him in the head. Either by this punch of by the leakage of a virus from Zero's capsule nearby, the Maverick Virus was transferred directly to Sigma, infecting him.

Following the battle, Sigma ordered the rest of his unit to retrieve Zero's body and contact Dr. Cain for study, limping away in anger. He was later repaired and reassumed control of the Maverick Hunters as usual. However, the Maverick Virus still lingered in his mind - while Sigma's highly-advanced circuitry kept him sane for a while, his thoughts steadily became more and more Maverick over time. The Maverick Virus fused with his mind, mutating into an even more dangerous virus called the Sigma Virus which he held complete control over. Despite this, Sigma seemed to remain stable for months after the lab incident, but still considered the role of Reploids in the world; for example, he noticed the father of all Reploids, Mega Man X, and his dedication to justice, and began wondering how he himself could change the world. Eventually, Sigma came to the conclusion that humanity were merely holding Reploids back from achieving their true potential, and secretly began organizing an anti-human rebellion.

Mega Man X
Without warning, one day Sigma finally puts his plans into motion and leads a mass revolt, declaring war on the humans and all Reploids who support them. Many Maverick Hunters follow Sigma out of loyalty - others, such as Storm Eagle, by force. This leaves Zero, who by this time has reformed and become a powerful Maverick Hunter, in charge of the organization. X agrees to join Zero in the fight, and X and Zero battle through Sigma's Maverick regime together. It is X who finally confronts Sigma alone in the Sigma Palace.

When met with X, Sigma first sends his pet, Velguarder, to test his power. Once it is destroyed, Sigma personally confronts him wielding his beam saber. After his first body is destroyed, Sigma's leftover head attaches itself to a giant canine body called "Wolf Sigma". This form can only be damaged by targeting its head. It should also be noted that, in the SNES version, Wolf Sigma is completely immune to the Hadouken, being only vulnerable to fully charged X-Buster shots and Rolling Shield.

After his defeat, Sigma laments X's decision for siding with humanity, believing that Reploids would have prospered without humanity holding them back. He then perishes in an explosion along with the rest of his fortress. Although his physical body is scrapped, Sigma's "soul" (the Virus) survives. After the end credits roll, Sigma's face appears on a blue monitor and warns X that he will be back.

Sigma's role in the remake Mega Man Maverick Hunter X is relatively unchanged. He is vocal about his desire to unleash X's full potential, believing him to be the future of all Reploids rather than an obstacle. His final battle also has some differences: his head willingly detaches from his body to enter Wolf Sigma, he has no lines during his death nor in the credits, and he falls into the tower below during his death animation. Besides the main game, Sigma also features heavily in the OVA The Day of Σ, and Sigma effectively acts as Vile's commanding officer in the non-canon Vile Mode, ordering him to challenge his lieutenants to quell their rioting. After Vile destroys them, Sigma invites Vile to his base to see his power up close. However, Vile is unable to get the chance to fight Sigma before being defeated by X and Zero. Sigma finds Vile's broken body afterwards, and questions him in regards to what his motives were. When Vile admits that he isn't sure himself, Sigma orders his men to extract Vile before departing.

Mega Man X2
Just as he promised, Sigma secretly returns and reassumes leadership of his Maverick forces thanks to the intervention of the fanatical X-Hunters. He gains a new body with the help of Serges specifically, becoming "Neo Sigma". He also oversees the "Unification" plot, which focuses on capturing and re-assembling the remains of Zero, whom he plans to reprogram as a loyal killing machine.

After Agile's defeat, marking the end of the X-Hunters, Sigma reveals himself in the Central Computer Stage. Sigma appears alongside Zero to battle X; if X successfully collected all of Zero's parts from the X-Hunters, it will be a Fake Zero whom the real Zero easily defeats for him. If not, X will be forced to fight his friend and incapacitate him, bringing him back to his senses. Afterwards, Sigma announces that he "knows Zero's secret", and that he's destined to serve him. He then flees to the depths of the Central Computer, and Zero opens the way for X to follow him for the final battle.

Neo Sigma battles X with arm-mounted claws and electricity attacks similar to Electric Spark. After this first body is destroyed, he assumes his true form, that of the Sigma Virus; this marks the first time Sigma's viral form is seen, as the final boss of the game. X defeats the Sigma Virus, causing him to apparently disappear as the Central Computer explodes. Before he disappears, he taunts X that he will return again and cryptically refers to Zero as "the last of the Doctor's creations."

While Neo Sigma is depicted wearing a cape in official art and the opening of the PSX/Saturn version of Mega Man X3, he is never seen wearing it in his in-game sprites.

Mega Man Xtreme
Sigma uses a hacker named Techno to hack into the Mother Computer of the Maverick Hunter base. Noticeably more confident than previous times, Sigma taunts X after he is beaten the first time and even calls his defeat at the end of Normal Mode merely a retreat. It is only when he is defeated on Hard Mode that he seems to succumb to X. However, he implies in the ending that he will return as long as not only Mavericks still exist, but also as long as humanity retains evil minds.

In battle, Sigma has the same forms and abilities from the first Mega Man X, complete with the same weaknesses. However, Wolf Sigma can also be damaged by normal X-Buster shots this time.

Mega Man X3
Sigma resumes his threat when he infects the Reploid scientist Dr. Doppler, who has developed a revolutionary new anti-virus to repel Sigma's influence. He uses Doppler to stage a revolt in Dopple Town, a utopian city established on the principles of peace and composed of reformed Mavericks. In the Saturn version, part-way through the game a cutscene plays showing that Dr. Doppler is building Sigma a formidable new body with the latest in Reploid technology.

Once Dr. Doppler is defeated by X, he feels remorse for his Maverick actions and reveals Sigma's hideout under Doppler Laboratory. Sigma is hunted by X and battles him using a shield and flamethrower, but is defeated. Sigma then emerges in a heavily-armored form, "Kaiser Sigma" (カイザーシグマ), to fight him again.

Upon this body's destruction the Sigma Virus itself appears in the form of a wire-frame head, identical to its appearance in Mega Man X2. He threatens to possess X's body to replace his own. After chasing X through the flaming wreckage of the base, Sigma finally corners him. If Zero did not end up critically wounded at any point during the game, which would have forced him to leave the war to X as he was repaired, Zero appears with Doppler's anti-virus on his saber to erase Sigma for the time being. If Zero did end up wounded, including the method by which X receives the Z-Saber, Doppler appears with the anti-virus on his own body and sacrifices himself to destroy the Sigma Virus.

Mega Man Xtreme 2
In Mega Man Xtreme 2, Sigma appears in Extreme Mode after the defeat of Berkana or Gareth (depending on if the player is X or Zero), where it's revealed he had planted the seeds leading to the Erasure incident. In a rare instance, he is pleased that X defeated Berkana, claiming that her greed went much too far.

Sigma's first form is the same as his Neo Sigma body from Mega Man X2, but his second form is "Sigma Beast" (シグマビースト), a large Ride Armor. The color of his second form is initially orange, but later alternates between blue and red, which can only be damaged by X while he is blue, and by Zero when red. His weak point is the large lion-like face in his chest.

Mega Man X4
Sigma was thought to be suppressed forever after receiving the anti-virus created by Doppler, but he still manages to return. Assuming a Grim Reaper-like appearance, Sigma meets with General, the leader of the benevolent militia Repliforce. He provokes feelings of doubt and paranoia of the Maverick Hunters within General, revealing that Sigma's latest plan is to conspire against Repliforce and turn it into a rogue enemy of the world.

In the game's inciting incident, Sigma orders Magma Dragoon to bomb the airborne metropolis Sky Lagoon, killing thousands and causing a rift to form between Repliforce and Maverick Hunters. This rift quickly becomes an all-out war as Repliforce refuses to cooperate and even stages a coup d'état to make a new land for Reploids. During the Great Repliforce War, Sigma allows Magma Dragoon to finally get his chance at a fight with X and Zero, and also has Cyber Peacock and Split Mushroom fight them to test their capabilities. He also sends a sleeper agent, Double, into the Maverick Hunters' ranks, meant to strike when the time is right to destroy the 17th Unit and damage X's trust.

With in-fighting between the Maverick Hunters and Repliforce threatening to destroy them both, Sigma increases his influence over General's paranoia. General, formerly an outspoken advocate of Reploid independence, loses all faith in peace and prepares a satellite attack on the humans, which is Sigma's real intention. Fortunately, X or Zero storm the Final Weapon and stop General before he can turn the weapon's sights on Earth - however, the weapon begins to prepare for attack anyways, revealing that someone else is controlling it.

In the end, Sigma is revealed as the true mastermind. In Zero's story, Sigma reveals a portion of Zero's past involving their first encounter. He then battles X or Zero himself, armed with a new body and a scythe with a beam blade. Once this body is destroyed, he attacks again with a collection of several bodies, including Gunner Sigma (シグマガンナー) and Earth Sigma (アースシグマ). After this ultimate defeat, Sigma reveals that the Final Weapon is still ready to strike the Earth - fortunately, General sacrifices himself to stop it from firing. Despite this apparent defeat at the Hunters' hands, Sigma still succeeded in part by destroying Repliforce, and by causing X and Zero to begin doubting their own morality.

Mega Man X5
It is some time before Sigma resurfaces again. During his absence, Sigma traces Zero's origins to Dr. Wily, and hatches a plot to unlock Zero's "true" power — the Zero Virus. The first part of his plot involves hiring the mercenary Dynamo to sabotage the space colony Eurasia with a virus and prepare it to crash into the Earth. Once Dynamo is in position, Sigma reveals himself to the Maverick Hunters and heads towards a giant statue while infecting new Mavericks along the way. He is confronted by one of the Hunters, X or Zero, and injures them, while the other shows up later and successfully defeats him.

However, upon Sigma's loss, the Sigma Virus begins spreading all over the Earth faster than ever. The entire world is thrown into a panic, but to make matters worse, Dynamo sends the Eurasia colony on a collision course with the Earth too. This combination overwhelms the Maverick Hunters, and regardless of their efforts, large pieces of the colony still hit the planet, causing massive damage and revealing a new virus called the Colony Virus. This virus mixes with the Sigma Virus, synthesizing the Zero Virus and forming an energy-based area called Zero Space wherein Sigma hides.

This was part of Sigma's ultimate goal: to form the original Zero Virus, intending to make Zero "remember" who he is and revert to his original, intended programming. Despite the creation of the Zero Virus and the ensuing battle between X and Zero, Sigma ultimately fails in this endeavor, as both X and Zero still manage to defeat him in both of his new bodies, Psycho Sigma (サイコシグマ) and the gargantuan Final Sigma W (ファイナルシグマW).

In the aftermath of the battle, Zero's severed torso lies dormant on the floor. X picks Zero up and tries to get him to respond. The remains of Sigma's head lying nearby suddenly become active and fire an energy laser directly at X and Zero, hitting both of them in the chests. As Zero's body falls from X's grasp, he manages to fire one last blast from his Z-Buster and finishes Sigma. X is critically wounded in the battle, both from the battle itself and Sigma's final attack (the force of the laser being so strong that it ripped through his entire lower half and arms), but he is miraculously repaired (the cutscene suggesting that it may have been by Dr. Light's hologram) and returned to the Hunters.

Mega Man X6
Three weeks pass as the human population moves underground to survive the devastation and pollution left by Sigma's last attack. Meanwhile, a renegade Reploid scientist, Gate, plans to use a newfound power known as the Nightmare to create a world of only the highest, most powerful Reploids. However, as a failsafe he also uncovers Sigma's remains and rebuilds him, keeping him in his Secret Lab as a last resort weapon. Due to the stress of spreading the Sigma Virus during the "Colony Incident", Sigma's programming is damaged beyond what Gate can repair, and he is reduced to a half-conscious and barely-functioning wreck of his former self. He seems to remember little more than his rage toward X and Zero and speaks in a garbled voice, rarely able to form sentences. Additionally, Gate's work on Sigma's new body is unfinished by the time he appears.

Upon his defeat at the hands of the Maverick Hunters, Gate reluctantly releases Sigma to attack them. However, Sigma refuses to cooperate and spitefully kills Gate before challenging the Hunters to face him deeper within the Secret Lab. They pursue him and he attacks using his incomplete body, but is defeated. After this first form, Sigma emerges as a gigantic unfinished body with a skeletal appearance named Hell Sigma (ヘルシグマ), and is again destroyed.

Mega Man X7
After hearing of Red Alert's success in recovering the Virus Program Σ-02 from Cedar and Aluce, Sigma decided to get in contact with the vigilante syndicate for his next plan. Sigma met with Red, this time under the alias of "The Professor", and informed him of his plan to use the mysterious Reploid Axl to copy the DNA programs of X and Zero for immense power boosts. Before putting this plan into motion, he proved his worth to Red by using the DNA Axl took from other Reploids to give power boosts to Red and his colleagues first. Many of the Reploids whom Axl was made to target to make DNA powerups were mostly innocent, irking Axl. This ended up helping him realize something was going on, and it prompted him to seek asylum with the Maverick Hunters.

This forced Sigma to find a way to bring Axl back into Red Alert's service. He secretly infected the eight Red Alert generals, blackmailing Red into following his orders and challenging the Maverick Hunters for custody of Axl. Once all eight of Red Alert's generals were defeated by the Hunters, Red leaked them the location of the Crimson Palace, which they then infiltrated. The Hunters fought Red, but he self-destructed out of grief and shame for his actions, and in order to keep himself from being further manipulated by Sigma.

Afterward, the Hunters continued on to confront Sigma, who was excited to see his old rivals again. After they defeated both of his forms, Sigma reappeared once more (appearing similar to his first form in Mega Man X6) and punched Axl through a wall before vowing to X and Zero that he would again return in a new body. Red, who had presumably died, suddenly reappeared, and Sigma attempted to possess him and regain his strength. However, Red revealed himself to actually be Axl copying Red's form, who then proceeded to blast Sigma out of a window.

Unusually for most games in the X series, Sigma is never identified by name.

Mega Man X8
Sigma reappears for presumably the last time during the Jakob Project. A new generation of Reploids goes into production, each containing advanced Copy Chips that give them the ability to shapeshift along with complete immunity to Maverick compromises such as the Sigma Virus. X witnesses this first-hand when an elevator car full of Reploids crashes at the Jakob Orbital Elevator, and he approaches only to find the car is full of Reploids who copied Sigma's durable frame to survive the impact. Lumine assures X that it is a safe process, but X is still bothered by it.

Soon after New Generation Reploids begin rioting across the globe, Alia discovers that their Copy Chips also bear Sigma's program - not just his body's DNA program, but his actual data as well. It is then revealed that Sigma is responsible for instigating the latest Maverick revolt, and that the New Generation Reploids are well-aware of his influence and entirely willing to work for him. Their relationship is compared to that of a father and children.

After the Maverick Hunters defeat the eight major New Generation Reploids, Sigma personally contacts Hunter Base. He tells them that he and his kind are fleeing to space to leave the old world behind, before challenging them to come find him via Jakob. Once the Hunters arrive in Gateway, they confront what appears to be Sigma himself - however, defeating it reveals it was just a Copy Sigma.

Alia manages to locate a Sigma Palace built on the surface of the Moon, and the Hunters head there. Once one of them reaches his throne room (the other having been separated by Vile's attack), Sigma affably welcomes them. He tells X his unique personality is what inspired his Maverick ambitions, thanks Zero for giving him the Maverick Virus in the first place, and calls Axl a worthless prototype. Sigma then strikes, and is able to immobilize the Hunter as he gloats in victory. However, the other Hunter arrives just on time to save their partner, and the two work together to defeat Sigma. The Maverick explodes, reduced to a pile of rubble.

Suddenly, Lumine appears and congratulates the Hunters for defeating Sigma. He then explains that he was using Sigma's ambition to help advance the New Generation Reploids' move to space, and now he can dispose of the older Reploids such as X, Zero, Axl, and even Sigma as well, for they are no longer needed in the new world. The Hunters manage to defeat both of Lumine's forms. Before his death, Lumine makes a startling claim: according to Lumine, Sigma was destroyed for good and not returning.

It is believed that the reason Sigma would not regenerate within another body after his final defeat is that he was on the Moon, where due to the lack of available bodies to possess, the virus might have weakened in the vastness of space and got completely deleted (although he was also defeated in space in Mega Man X4, however, that time it was on a large satellite; and the fact that Sigma didn't seem frustrated about this defeat implying that he could escape). In addition, Sigma before dying for the final time asks in disbelief how X, Zero, and Axl could even beat him, before promptly saying "you couldn't...", implying alongside Lumine's prompt crushing of his remains head that Lumine may have weakened him somehow. There was also a lack of spare bodies, and his last body was hardly finished. However, his DNA is included in every copy chip of all the next generation Reploids so, technically, all New Generation Reploids are "infected" with the Sigma Virus, and possibly Lumine as well; meaning Sigma may return one day.

Post-Mega Man X series
Although Sigma himself never returned in person, the Sigma Virus still lived on and would plague the world until the discovery and creation of the Mother Elf from enzymes in Zero's body. X then utilized the Mother Elf to fully neutralize the Sigma Virus, bringing a permanent end to the Maverick Wars that Sigma had started. Unfortunately, this series of events would later lead to the far more destructive Elf Wars, due to Dr. Weil's radical agenda to punish the Reploids for the damage caused in the many conflicts during the Maverick Wars.

Other game appearances



 * Sigma shows up briefly on the green monitors of the Mega Man X: Command Mission opening cutscene, which shows some scenes from the Mega Man X7 ending.


 * Sigma's first form from Mega Man X5, Psycho Sigma, appears in Mega Man X Legacy Collection 2 ' s X Challenge mode, in which he is fought in stage 6-3 alongside Zero Nightmare.


 * In Rockman Xover, Sigma, along with Dr. Wily and other villains from other Mega Man series, join forces to wreak havoc. He is the boss from World 2 and World 7, and appeared in several Battle Memories.


 * Sigma appeared in Street Fighter × All Capcom.


 * Sigma, with his appearance from Mega Man X4, is one of Chuck Greene's costumes in the Dead Rising 3 DLC mode "Super Ultra Dead Rising 3' Arcade Remix Hyper Edition EX Plus α". He attacks with his scythe and laser eye, and has a special attack called "Sigma Crusher".


 * Sigma appears in Project X Zone 2, the sequel to the first game (where he was only mentioned by name), as an enemy unit in his Mega Man X4 form.


 * Sigma appears as a Spirit in Super Smash Bros. Ultimate with his appearance from Mega Man Maverick Hunter X. He is an Attack-type Primary Spirit in the "Ace" class. In Adventure Mode, Sigma is found guarding Mega Man. The playable character representing him is Ganondorf, and the theme music is X vs. Zero from Mega Man X5 and Mega Man X6, alluding to how Sigma largely instigated X and Zero's fight in the former game.


 * Sigma appeared in the illustrations from some action cards in TEPPEN.


 * Sigma appeared as an event character in the Japanese version of Zombie Cafe.


 * Neo Sigma appeared as an event character in Dragon Poker.


 * Sigma appeared as an event character in Elemental Story.


 * Sigma appeared as an event character in Tower of Saviors.


 * Sigma (written as SIGUMA and SIKUMA in the game) appeared as a boss in the Rockman X DiVE event from the Taiwanese version of Shironeko Project. He is fought in his Sigma Head form from Mega Man X5.


 * Sigma would have appeared as a playable character in the cancelled game Capcom Super League Online.

Marvel vs. Capcom: Infinite
Sigma appears with his original form from Mega Man X as a downloadable playable character, and appears in the story mode as part of an antagonist tag team alliance with Marvel Comics' Ultron. His default design is derived from his appearance in Mega Man X, though he implies that it was only one of several forms he had taken and was not particularly fond of this form. His Premium outfit is his Final W from Mega Man X5. His move set makes use of his beam saber with specials like Arc Divide, swinging in multiple broad arcs, each slash producing patches of corrupted data that further damages the enemy and Straight Divide, based on his airborne dash from Mega Man X. New to his arsenal is Reflector Magnum, which fires missiles from his fingers that bounce off of floors and a Counter Teleport where he teleports behind the opponent after parrying a physical attack. It can also be used to reflect projectiles. His Hypers are Doom Buster, firing a large ball of fire in front of him, Blazing Line, which has him jump into the air and shoot a traveling laser that rakes the ground with large explosions and Rave Divide, a series of powerful slashes with his saber. His Lv. 3 Hyper is Final Sigma wherein he grabs the opponent's face and transports them into a digital world where wireframe copies of himself unleash a barrage of brutal attacks, ending with them skewering the victim and returning them to the physical world, his hand still grabbing their face. When Thanos and Ultron go to Abel City to obtain the Infinity Stone that was with Sigma, Ultron and Sigma decided to form an alliance, with Sigma allowing Ultron stab his body with his hand (with Sigma suggesting they do so via that method instead of Ultron's original plan of utilizing violence alone), armed with Reality Stone to upload his mind into Ultron’s body, becoming a singular being known as Ultron Sigma and betray Thanos as part of their mutual agenda to wipe out all organic life. After defeating Thanos with their combined powers and taking the Space Stone from Thanos, Ultron Sigma using both Reality and Space Stones to transform Ultron’s body into their current primary appearance of Ultron Sigma form, and fuse their worlds. After Dante managed to trick Ultron Sigma into destroying themselves with the Soul Stone, they ended up mutating into a massive form known as Ultron Omega from that Stone’s full potential side-effect, before being finished off for good by the heroes as well as X and the Infinity Buster.

Mega Man (Ruby-Spears)
Though not a starring character, Sigma was referred to in the episode Mega X of the Ruby-Spears Mega Man animated series, where X, Vile and Spark Mandrill go back in time. His profile, shown by X, shows him in his original body and mistakenly spells his name as Cigma. The only line Sigma has is when Vile contacts him near the end of the episode — a short praise: "Well done, Vile."

Mega Man (Archie Comics)
Sigma is the main antagonist from the Mega Man X era and the Worlds Unite crossover.

Rockman X
Sigma starts a rebellion against humans, beginning by killing a Maverick Hunter, Kyle, that was against him, and convincing various Hunters to join him by saying that Reploids are superior to humans and they should no longer take orders from them.

When Spark Mandrill was giving up the fight against X with fear and saying he would tell all of Sigma's plans to him, Sigma uses a giant hologram of himself (via a flying mechanism) to kill Spark Mandrill and talk with X. Before the fight against Armored Armadillo, it's shown that Sigma scarred Armadillo's left eye because he didn't follow his orders to defeat X in the forest. After defeating the eight Mavericks, Sigma appears and challenges X to fight him in his fortress. Although the battle is difficult, X manages to defeat Sigma. Sigma then uses a giant body that is too powerful for X, but he manages to destroy him by crashing his fortress.

In Rockman X2, Sigma is shown to have survived the event and starts working on a more powerful body. Still, after the defeat of eight Mavericks and the X-Hunters, Sigma is defeated by X and Zero.

In Rockman X3, Sigma took control over Dr. Doppler to start another Maverick uprising, using him to infect several Reploids with "Worms" and create a more powerful body by analyzing X. He has a fierce battle against X, but he is eventually defeated by X with Zero's saber. However, he reveals a more powerful body, Kaiser Sigma, and manages to destroy X. To Sigma's surprise, X regenerates and his armor became golden, and he uses his new power to destroy Sigma. Sigma also appeared disguised as an angel when a worm infected by X, who told X the latter can't enter paradise due to "sins" in an attempt to cast doubt into him, although X eventually saw through the illusion and shot Sigma.

Sigma only has a cameo appearance in the Rockman X4 manga, being mentioned by Double in the last chapter where he makes clear he had been the true cause of the Great Repliforce War.

Irregular Hunter Rockman X
Sigma appears as the leader of the Maverick Hunters. When Storm Eagle challenges X to a fight, Sigma appears and stops Storm Eagle without destroying him, giving him a lecture and the Storm Tornado to X. However, when Zero starts acting strangely, Sigma tells Armored Armadillo that Zero became a Maverick and orders him to destroy Zero. Later, Vile escapes from his cell and takes four Hunters to work for him. After the defeat of the four Hunters, Sigma orders X to go after Zero and destroy him, but Zero reveals Sigma's involvement in the events, and they are attacked by Vile, Storm Eagle, Armored Armadillo, and Boomer Kuwanger. After the defeat of the latter three, Vile escapes and Sigma decides to destroy the base of the Maverick Hunters. After talking with A-1, X and Zero go after Sigma, who started attacking the city with several Mechaniloids. Sigma appears to face the two Hunters, but after being hit by two charge shots at the same time, He calls Vile and shows that they took A-1 as hostage, taking advantage over X. Unable to see X taking so many hits, A-1 releases himself and runs to Sigma, self-destructing near him. Sigma is stunned by the explosion, and X, angry and sad for A-1, fires a large charge shot, and Zero also fires one at the same time, but Vile saves Sigma and they escape. While leaving, Sigma tells them that he will surely conquer the world.

Rockman X The Novel: Irregulars Report
Sigma was created by Dr. Cain as his masterpiece. However, upon investigating the ruins where he fought against a Maverick Zero, he discovered the truth about X and the Reploids and was filled with hatred for being simply a copy of X, which led to his plan to rebel against humanity and test X's abilities.

Trivia



 * Interestingly, Sigma was designed by merging a team of three other early villain designs created by Inafune during the early development of the first Mega Man X game. The same designs were later used again as the basis for the X-Hunters in Mega Man X2.
 * His appearance has a close similarity to both M. Bison and Sagat, both of whom are major villains of the Street Fighter series. He also resembles Braiking Boss, the main villain from Casshern. Due to this, some fans believe that Sigma was based on one of them. The resemblance to Bison is even referenced in Project X Zone 2.
 * During a 2008 question and answer session, when asked if Sagat served as inspiration for Sigma, Keiji Inafune stated that Sigma (and Zero) represent the idea that nothing is absolute.


 * Sigma (Σ) is the 18th letter of the Greek alphabet and is used in mathematics to denote series, finite or infinite sums of variables. The Σ symbol is often seen on Mavericks from Sigma's army. Interestingly, the Sigma (Σ) letter was originated from the Phoenician letter Sheen (W), which is similar to the English letter "W".
 * On a related note, commentary by Keiji Inafune on concept art for Sigma during Mega Man X made it clear that Sigma's connection to Dr. Wily was intended from the start.


 * There are four variations on the way Sigma got the scars on his eyes:


 * Zero damaged Sigma's face when they fought (as shown in an animated cutscene in X4).
 * X damaged his face in The Day of Σ. Ironically, after Sigma's head merges with the wolf robot in the game, his face resembles his face deformity received against Maverick Zero.
 * Sigma did it to himself after going Maverick in the manga, by using the heat of the "weapon" (his fingers) he used to shoot Kyle to make the scars. Something similar was observed in the 2017 novelization while contacting X and Zero.
 * The Mega Man X 2017 novelization indicates that Sigma's trademark scars were self-inflicted, as when communicating with Zero, the newly-Maverick Reploid is seen putting his right hand near his bloody eyes, with the fingers having traces of blood on it.


 * The penultimate cutscene of Zero's game in Mega Man X4 reveals Sigma's pre-maverick design, which lacked his facial scars and the spikes featured on his wrists and knees featured on his original design. This design would be re-used for his pre-maverick appearances in Maverick Hunter X, with the addition of slightly redesigned wrist guards. The single-frame depiction of this battle from Mega Man X5 however discards the redesign, instead of using the standard Maverick Sigma design with the exception of the facial scars, which was likely a design oversight.


 * Sigma's normal form in Mega Man X8 is a redesign of his armor from the first Mega Man X.


 * If Sigma's name is entered as the player's name in Mega Man 9, the game will erroneously say that it is "inappropriate language", and changes it to "Si**a".


 * Although Sigma is never seen or mentioned in the Mega Man Zero series he does make a cameo in the Rockman Zero manga, though the Sigma Virus is mentioned in Mega Man Zero 2 and Mega Man Zero 3 and less notably, in the first game, his symbol is seen (though difficult to spot because of the background partially covering it) in the very left end of the chamber where Zero was resting.


 * In Mega Man X8, the copy of Sigma is weak against Dark Mantis' weapon, while the real Sigma is weak against Optic Sunflower's weapon. Both weapons are based on darkness and light, respectively.


 * In Ruby-Spears Mega Man, X mentions to Dr. Light that lightanium is worth billions in the future, which can fund Sigma's rebellion.


 * Oddly, only the first and third Mega Man X games have official artwork for both of Sigma's forms.


 * In the original Mega Man X, Sigma berates X as he realized why Zero counted on him, while in the remake, he does no such thing, but commends X's ability to finally reach him.


 * In the original Mega Man X, Wolf Sigma has floating hands. In Maverick Hunter X, however, Wolf Sigma has arms and hands.


 * The Dark Dragon's attack pattern from The Legend of Zelda: Oracle of Seasons is reminiscent of Wolf Sigma's attack pattern. Both the Mega Man X games and Oracle of Seasons were developed by Capcom.


 * In the August 9th prototype for Rockman X3, Sigma's design is closer to that of the first two games. The redesign in later builds and the final game is unknown, but it takes up more memory space than the prototype's graphics.
 * According to the Rockman X Saikyou Daizokan, Wolf Sigma is an incomplete Reploid unto itself, which Sigma then assumes control over: "A yet incomplete extra large wolf type repliroid. Sigma's head takes over the functions of the brain, but because of its excessive size, the feet and arms require more development time to be completed. Once the super-powered repliroid is completed, Sigma will be unstoppable in his plan for world domination!"