Sigma Head

Sigma Head (シグマヘッド) is the boss from the Opening Stage in Mega Man X5. He wounds the character opposite to the one the player chooses (he will destroy X's Fourth Armor if the player chose Zero, or destroys Zero's buster if the player chooses X) and then hides inside a statue under construction.

History
A few months after the Great Repliforce War, Sigma hatched up a new plot to decimate humanity and create more Mavericks by spreading the Sigma Virus all over the planet. To achieve this end, he has the bounty hunter Dynamo blow a hole in the colony of Eurasia and send it on a collision course toward Earth. Meanwhile, Sigma ambushes the Maverick Hunters at a city in northern Brazil in the form of a giant head.

Although X and Zero manage to defeat Sigma in this form and cause it to explode, they did not realize until it was too late that Sigma lost to them on purpose. The explosion caused by his defeat causes the virus to spread from South America all over the Earth to different continents, resulting in the majority of Hunters becoming Mavericks themselves. Even seaside Reploids such as Tidal Whale were infected by the virus.

Strategy
In this giant head form, Sigma releases small energy spheres from his mouth or his eyes. They will be a barrage of blue spheres if he releases them from his mouth, and two green spheres that track the opponent if released from his eyes. He can also attack by ramming toward his opponent, with his reach ending at the opponent's side of the arena. After losing half of his energy, he will fire a large purple energy beam from his mouth. His only vulnerable area is the cannon's lens that only appears when he opens his mouth to attack.

Shironeko Project
Sigma Head (called only as Sigma and Sigma Virus in the game, misnamed as "Siguma" and "Sikuma" when romanized) is Sigma's form in the Rockman X DiVE event from the Taiwanese version of Shironeko Project. Sigma is the final boss from the Rockman X DiVE event, appearing with small Sigma Heads (possibly based on the Sigma Virus enemy from Mega Man X5) assisting him. Sigma starts with four small Sigma Heads that form a barrier, preventing the party from approaching him. After breaking the four miniatures, the barrier disappears and Sigma summons more small Sigma Heads to assist him, and will occasionally summon some of the robotic enemies from Shironeko Project. Both the large and small Sigmas attack with lasers and energy spheres. Defeated miniatures are replaced by new ones after some time.

During the event Sigma also appeared as a co-op boss with three difficulties, the first being similar to the story event, the second having two large Sigma Heads (the second appearing after breaking the barrier) that must be defeated, and the third having one head followed by three heads. In the third difficulty, players must not take long to defeat the set of three large heads as, like the small ones, after some time a new head will replace the defeated one.

Other media
The Sigma Head appears in the Rockman X5 manhua serving the same role as in the game, only fighting both X and Zero.

Trivia

 * Sigma Head's boss theme is the same one that is used during the first phase of the Sigma boss fight in the game.