Sigma

"Listen well, X. There are times when we Maverick Hunters cannot afford to hesitate in pulling the trigger. We must become both sword and shield to those who cannot protect themselves - that is our sworn duty. Never forget that!"

- Sigma, as "Commander Sigma"

"It's not over yet, X! The battle! The angst! The destruction! I will make you understand the true potential of Reploids!"

- Sigma as a Maverick

Sigma (シグマ), formerly referred to as "Commander Sigma", is the main antagonist of the Mega Man X series. He serves as the final boss in all games in the series except Mega Man X: Command Mission (where he makes no appearances) and Mega Man X8 (appearing as the final boss if the game is played on easy, and as the penultimate boss if the game is set to normal or hard difficulty, since Lumine is fought on normal or hard).

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 in 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.

Before Mega Man X
As the former leader of the Maverick Hunters, Sigma was originally the most advanced Reploid of his time. Dr. Cain implanted the latest circuitry designs into Sigma, which were theoretically supposed to prevent him from going Maverick. He was placed in charge of the 17th Elite Unit of the Maverick Hunters, and led them to victory multiple times.

One day, the Maverick Hunters received reports of an overwhelmingly powerful "Red Maverick" that has destroyed an entire squadron of Hunters (the Gamma unit). While investigating, Sigma, along with his unit, tracked it down to an abandoned laboratory. Sigma then left to confront it alone and personally deal with it, in order to prevent other Reploids' deaths at the hands of the Maverick. 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 beaten badly by the maddened Zero, but managed to smash the energy crystal in Zero's helmet. The fight, due to taking place in close proximity to Zero's capsule, which was leaking trace amounts of the Maverick Virus, and the damage they already sustained in the battle, transferred the Maverick Virus directly into Sigma and Zero, infecting them. Zero lost consciousness, and Sigma limped away from the battle, ordering his men to take the body of Zero back for study. The virus had different effects on them: Although the virus corrected the flawed programming in Zero, it began to mutate and adapt to Sigma's own programming, eventually fusing entirely with his mind to form the Sigma Virus, which allowed him to manipulate and control Reploids, bending them to his will. After this, he began to feel that humanity was keeping Reploid potential down, and decided to revolt against humanity.

The Day of Σ
Sigma appears in a short unlockable OVA in the game Mega Man Maverick Hunter X for the PSP entitled "The Day of Σ". It shows the Maverick Hunters prior to the events of the game under Sigma's command, before he ultimately revolts. It should be noted that the events in this OVA coincide more with the canon of Mega Man Maverick Hunter X than with that of the original game.

Mega Man X/Maverick Hunter X
"You say you believe in justice. But that's not true. You've only been led to think that. Nothing more!"

- Sigma

Sigma's advanced circuitry seems to do its job, as he seemingly remains stable for months after the lab incident. He gradually succumbs to the virus inside of him and grows more volatile. Without warning, he leads a mass revolt among the Reploids, declaring total war on humanity, which is seen depicted in The Day of Σ. Many Maverick Hunters follow Sigma out of loyalty, leaving Zero, who by this time was reformed and had become a powerful Maverick Hunter, in charge of the organization. Mega Man X, the last creation of Dr. Light and the base model for every Reploid, decides to join Zero as thanks for saving his life. X and Zero battle through the Maverick regime, and it is X who finally destroys Sigma, who laments X's decision for siding with humanity, feeling that Reploids would have prospered without humanity holding them back. But, though 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 will first send his pet, Velguarder, to test X's worthiness, before confronting him wielding a beam saber. Sigma's attacks consist mostly of dashing and wall jumping. He also fires lasers from the crystal in his forehead. After defeating his first form, Sigma's head attaches itself to a canine-like, giant body called "Wolf Sigma". This form attacks with claws, electricity, and fire, and 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, and is only vulnerable to fully charged X-buster shots and the Rolling Shield.


 * In Mega Man X and its remake Mega Man Maverick Hunter X, it is shown that coupled with his goal to exterminate humanity, Sigma is also obsessed with unleashing X's full potential, believing him to be the future of all Reploids. His death is also slightly different. Aside from his lamenting about X's decision for siding with humanity being cut, Sigma also slips and struggles to get back up, eventually inching closer to X (presumably as a last attempt at killing X), only for his wolf body's explosions to worsen and have him fall to his death. Besides his actions in the main game, he also effectively acts as Vile's commanding officer in Vile Mode, telling him to challenge his lieutenants. After Vile successfully does so, he also invites Vile over to fight him. However, Vile was unable to get the chance to do so after being (barely) defeated by X and Zero. Sigma also finds Vile's broken body, and questions him in regards to what his motives were in the Maverick Wars. When Vile admitted that even he didn't know, Sigma then leaves him, although not before making sure to radio some of his men with the order to retrieve him.

Mega Man X2
Just as he promised he would, Sigma returns and assumes leadership of his Maverick forces. He gains a new body with the help of the X-Hunters, a trio of vengeful Mavericks who worship him, and becomes "Neo Sigma". He also oversees the "Unification" plot, which focuses on capturing and re-assembling the remains of Zero, whom he plans to reprogram to kill X. But, since X managed to recover all of Zero's remains from the X-Hunters, Sigma was forced to make a copy of Zero's body that was not unlike Zero's Black Zero form; however, Zero arrives and destroys it completely before it could battle with X. Sigma then announces that he "knows Zero's secret", and that he's destined to serve him. It's unknown what exactly this secret is, but the dialogue suggests that it has something to do with Zero being created by Dr. Wily, and the possible origins of the Maverick Virus as Sigma calls him "the last of the Doctor's creations". Sigma attacks in his latest form, armed with clawed knuckles, but X thwarts his plans and destroys him again. This marks the first time Sigma's viral form is seen, as the final boss of the game. Zero is then restored to his rightful place in the Maverick Hunters, and is given command of his own Special 0 Unit.

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.


 * Sigma has the same forms from the first Mega Man X, complete with the same weaknesses. However, Wolf-Sigma can also be damaged by normal buster shots, and can easily be defeated with rapid-fire turned on.

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. Prior to his reemergence, Sigma's bodies after they were destroyed were presumed to have been used for scrap. Sigma uses him to stage a revolt in Doppler Town, a Utopian city established on the principles of peace. Once Doppler is unmasked as a pawn, Sigma emerges in a heavily-armored form, "Kaiser Sigma" (カイザーシグマ). After his defeat for the third time, The Sigma Virus itself appears in the form of a wire-frame head, which is identical to it's appearance in Mega Man X2, and threatens to possess X's body to replace its 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 and utilizes the anti-Virus created by Doppler to erase Sigma for the time being. If Zero did end up critically wounded, including the method by which X receives the Z-Saber, Doppler uses the anti-Virus and sacrifices himself to destroy Sigma's programming.

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 was behind the Erasure incident. In a rare instance, he's pleased that X defeated Berkana, claiming her greed went too far.


 * Sigma's first form is the same from Mega Man X2, "Neo Sigma", 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 big 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. In the introduction of the game, Sigma, assuming a Grim Reaper-like appearance, acts as a double-agent between the Mavericks and Repliforce, a benevolent squadron of Reploids who assist the Maverick Hunters. Sigma's true motive is to conspire against Repliforce and turn it into a rogue enemy. He stages the bombing of the airborne metropolis Sky Lagoon, which plummets to the ground in the opening of the game.

With in-fighting between the Hunters and Repliforce threatening to destroy them both, Sigma increases his influence over the paranoid General of Repliforce. The General, an outspoken advocate of Reploid independence, loses all faith in peace and prepares a satellite laser attack on all humans, which is Sigma's real intention. Fortunately, X storms the space weapon and stops the General before he can turn its sights on Earth. In the end, Sigma is revealed as the true mastermind and battles X himself, armed with a new body and a scythe with a beam blade. Despite his defeat at X's hands, he succeeds in part by destroying Repliforce. In order to ensure both sides of the war were manipulated, he also had one of his agents, Double, infiltrate the ranks of the Maverick Hunters, and also seduced Magma Dragoon with the concept of power and a chance to fight X/Zero in exchange for instigating the war. He also had Cyber Peacock and Split Mushroom fight against X/Zero to test their power and capabilities. Eventually, he had Double fight X when he infiltrated the Final Weapon, and also ordered him to kill some members of the 17th Unit of the Maverick Hunters as their presence risked compromising Sigma's plans.


 * If the game is played with Zero, Sigma reveals a portion of Zero's past involving their first encounter that possibly made Sigma a Maverick.


 * Interestingly, this is the only game where all of Sigma's audio pre-battle quotes are threats ("Tremble with terror! YOU ARE FINISHED!", "Prepare to become space dust!", "Are you ready for your final journey?!" or "Are you ready for round two!?", the last of which is uttered during the transition between the first round and the second round). Also, despite being the final boss, his attack patterns in this game are simpler than some of the other bosses.


 * Sigma's subversion ultimately inciting the Repliforce rebellion under the rationale that humanity (and by extension, the Maverick Hunters) would ultimately destroy them or anything that isn't wholly subservient foreshadows Dr. Weil's conception, as the latter gives a similar description of humanity.


 * Sigma's grim reaper form is the only time in any X series game where the player cannot progress without using a special weapon; it can only be damaged by X's Rising Fire or Zero's Ryuenjin.


 * This is the only game where Sigma features four forms.


 * Sigma's final forms in this game are named Gunner Sigma (シグマガンナー) and Earth Sigma (アースシグマ) in Mega Man X Legacy Collection.



Mega Man X5
It is some time before Sigma resurfaces again. During his absence, Sigma traced Zero's origins to Dr. Wily, and hatches a plot to unlock Zero's "true" power — the Zero Virus — inside him. Sigma also adapts his programming to completely overwhelm the anti-virus; the Sigma Virus quickly begins spreading everywhere, turning peaceful machines and Maverick Hunters alike into violent servants of Sigma. The Maverick leader also enlists the help of a Reploid bounty hunter named Dynamo to crash the orbiting space colony Eurasia into the Earth, as well as to hinder the Hunters' plans of stopping the collision.

Sigma's ultimate goal is to mix his own viral code, which is spread across Earth, with the original Zero Virus, and become all-powerful. Sigma hesitates to kill Zero, intending to make him "remember" who he is and revert to his original, intended programming. He fails in this endeavor, and both X and Zero defeat Sigma's new body (which has been possibly powered and/or developed by Dr. Wily, although this may just be speculation) and his more gargantuan Final Sigma W (ファイナルシグマW) body. The speculation comes from these quotes from Sigma after the player defeats Sigma's first form and before the battle with the second form:

When playing as X:

"Here I am. You can challenge me at anytime. I have delightful news. I've recently acquired a new partner. He has been very supportive. He seems to have created quite a few robots. And he gave me the toughest body that you will ever see. You got here sooner than I expected, so it is not yet complete... But...it is enough to defeat you... He is an excellent partner... I believe you two know each other... In fact, he used to be a comrade of yours. He was very persistent about you... and that makes him very helpful to me. You see X, there is someone other than me...who hates you... Now feel our combined rage and die! ...But not before suffering horribly, hahaha! ...Goodbye, X!"

When playing as Zero:

"Hee hee hee hee. Calm down... I'm here. Let me tell you about an old man I met... He told me you are the strongest in the universe... Tell me, who is that old man? You should know him. Don't tell me you've never met him in your dreams. He seems very interested in you... as if he was your own father...!"

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 chest. 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 body and destroyed his entire lower half and arms), but he is eventually repaired (the cutscene suggesting that it may have been by Dr. Light) and returned to the Hunters.

During the first phase of the battle, Sigma largely uses similar maneuvers as in the first fight against him in Mega Man X, only using destructive waves instead of his laser sword.


 * Sigma's first form in this game is dubbed Psycho Sigma in Mega Man X Legacy Collection ' s X Challenge mode, in which he is fought in this state in stage 6-3 of Volume 2 alongside Zero Nightmare.

Mega Man X6
Three weeks pass as the human population moves underground to survive the devastation and pollution left by Sigma's last attack. Sigma himself is quickly resurrected by a renegade Reploid scientist, Gate, who was angry at the scientific community for rejecting his advanced Reploid designs. Gate becomes a Maverick after coming into contact with the Zero's DNA in the Eurasia remains, a piece of rubble that contains signatures of the Zero Virus. He then uncovers Sigma's remains, and proceeds to rebuild him. However, due to the stress of spreading the Sigma Virus during Mega Man X5's "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. Gate's work on Sigma's new body is unfinished by the time X or Zero confronts him at the end of the game. Although it seems as though Gate rebuilt Sigma merely as a backup plan to further his own goals, Sigma is quick to finish him off after X or Zero finally defeat Gate in battle, stating: "Oh please, I did not die, nor did I need your help. Now get lost!"

After X or Zero defeat his first form, he emerges as a gigantic unfinished body with a skeletal appearance named Hell Sigma (ヘルシグマ).

Along with his final lines before being destroyed, this is the only instance in Mega Man X6 in which Sigma speaks coherently. He seems to remember little more than his rage toward X and Zero and speaks in a garbled voice.


 * While the player only fights this "reborn" form of Sigma in Mega Man X6, a similar-looking one appears in the ending cinematic for Mega Man X7, as well as various cutscenes with Sigma and Red (this implies that Sigma may have been stuck in a worn down version of his body thanks to Gate).

Mega Man X7
Sigma reappears yet again, under the alias of "The Professor", to manipulate the Maverick hunting organization known as Red Alert. His plan was to use a mysterious young Reploid named Axl to copy the D.N.A. signatures of X and Zero and implant them into destructive war machines. The plan fails when Axl flees Red Alert and seeks asylum with the Maverick Hunters.

This prompts Sigma to find a way to bring Axl back into the Red Alert group. He secretly infected the eight Red Alert generals, blackmailing Red into following his orders and causing Maverick uprisings to draw in the Hunters' attention so that they could infiltrate the Crimson Palace. There, the Hunters confront Red, and upon being defeated he regrets being used by "The Professor" and sets part of the building to self-destruct, seemingly killing himself in the process.

Afterwards, the Hunters confronted Sigma, and once defeated, a reborn Sigma (who appears similar to his first form in Mega Man X6) punches Axl through a wall before vowing to X and Zero to again return in a new body.

Red, who had been defeated earlier and presumably killed, reappears, and Sigma attempts to possess him. Red is then revealed to be Axl taking on Red's form, who then proceeds to blast Sigma out of a window.

Unusually for most games in the X series, Sigma is not actually identified by name, even when he reveals himself.

Mega Man X8
Sigma's next plot was very complex, and it wasn't even him behind the entire scheme. It turned out that Axl was a prototype for a new generation of Reploids. These next generation Reploids possessed advanced copy abilities, and Alia discovered that the copy chips bore copies of Sigma's program among other powerful Reploids. Sigma instigated a new Maverick revolt with these new Reploids as the humans were evacuating Earth in hopes of finding a new home on the Moon. However, the revolt was stopped, Sigma was completely devastated, and the real mastermind, Lumine, the manager of the Jakob Project, was revealed and beaten. Before his death, Lumine made 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). 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.

Although Sigma himself was destroyed for good, 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, due to Zero being the original carrier of the Maverick Virus, which was then neutralized and brought about a permanent end to the Maverick Wars. Unfortunately, the discovery would also result in the start of the far more destructive Elf Wars due to Dr. Weil's radical agenda to punish the Reploids for the damages they caused during the Maverick Wars.

Other games

 * 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 appeared in Battle Memory.
 * 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 in name), as an enemy unit in his Mega Man X4 form.
 * Sigma appeared as an event character in Zombie Cafe.
 * Sigma appears as a Spirit in Super Smash Bros. Ultimate. He is an Attack-type Primary Spirit in the "Ace" class. In Adventure Mode, Sigma is found guarding Mega Man.
 * Sigma is a playable character in 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 moveset 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 airbone 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.

Appearance
Although Sigma has taken many drastically diverse forms throughout the X series, his humanoid design remains fairly consistent, resembling a giant bald human with a cleft chin and also possessing a scar over each eye (although the exact degree he was scarred varied by game, with most incarnations being medium length, Mega Man X8 having comparatively thin scarring, and Mega Man X3 having the scars literally surround his eyes).

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 typically looks down on others and does not hesitate to kill the weak, 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, consumed by the lingering thought of killing X. 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 hatred for X and Zero, Sigma once believed they were the future of all Reploids, and would lead them all into a new age. In addition, he also credited X's dedication to justice, as well as his being infected with Zero's virus, as being his primary motivation in his going Maverick, including what ultimately turned out to be his final battle on the Moon. 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.

In the remake Maverick Hunter X, however, he was shown to be a far darker and more evil character, where his own actions and motives were simply to cause as much carnage as humanly possible due to thinking that was the Reploids' true potential, and simply being curious about X's potential. In addition, the Vile Mode and to a certain extent the main game implies he holds absolutely no loyalty to his fellow Mavericks, and thus made his claims of doing his actions for the best interests of all reploids self-serving at best, as he had zero problem with his lieutenants being killed by Vile and even ordered him to take them out beforehand, and he also showed absolutely no sadness at X destroying Velguarder and if anything congratulated him. However, in the beginning of Vile Mode, Sigma when giving Vile the opportunity to leave regardless of whether he'll aid Sigma in his rebellion to defeat X or not specifically states that he "needed a Reploid who can go maverick of his own accord", implying the possibility that Sigma's becoming maverick was not entirely of his own choosing but due to external measures like in the original game.

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. 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 Sigma 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.

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 and Zero. He has a fierce battle against X, but he is eventually defeated. However, he reveals a more powerful body, Kaizer 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 only has a cameo appearance in the Rockman X4 manga, being mentioned by Double in the last chapter.

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 later 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.

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 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".


 * 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.


 * Sigma's Belial form from X8 bears a slight resemblance to Abyss (first form) from Marvel vs. Capcom 2.
 * 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 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.
 * Also, Sigma's "scars" in Mega Man X8 do not really look like scars, rather just lines running down his face.


 * 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. This creates some confusion as to why Sigma ever needed funds, as he shows to not have any necessity to have funds for anything.


 * 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.