Sigma (シグマ) is a major character and antagonist in the X era, serving as the final boss of most Mega Man X games. He is a highly-advanced Reploid who was originally built by Dr. Cain to lead the Maverick Hunters, but he developed a hatred for humans and threatened to take over the world. To make matters worse, his mind transformed into the Sigma Virus, a form in which he survived throughout the series even as his body was destroyed.
Though Sigma assumes different forms throughout the series, he always retains iconic features such as his bald head, glowing eyes, and a muscular figure with green and/or purple armor.
Sigma debuted in Mega Man X, where his backstory is described in the game's manual. Sigma was created by Dr. Cain to lead the Maverick Hunters in the fight against the dangerous Mavericks. He was Cain's magnum opus, being touted as the most powerful and intelligent Reploid of all time. He was given a prestigious SA rank and personally took charge of the 17th Elite Unit.[1] He initially took his post on March 16th, 20XX, effectively leading the Maverick Hunters for at least four months. On June 4th, 20XX, Sigma suddenly went Maverick, having secretly organized an anti-human rebellion using the Maverick Hunters as his army.[note 1] This is the initial incident of the game, forcing righteous Hunters such as X and Zero to go up against him and their former comrades to save humanity.
Sigma is only seen in-game once all eight Mavericks on the Stage Select screen are defeated: with the final stages open, Sigma's portrait is projected into the center of the screen. X does not encounter Sigma until Sigma Stage 4, the final stage of the game. After X climbs to the top of a vertical shaft, Sigma teleports in, wearing a cloak, and gloats that he could easily destroy X, but will leave him to his pet, Velguarder. Once X defeats it, Sigma returns and applauds his strength before discarding the cloak to face him with a beam saber. When Sigma is defeated in this form, his body is destroyed and only his head remains, which ascends into a giant wolf-like robot in the background: Wolf Sigma. When X is once again victorious and the robot begins to explode, Sigma mourns his failure in disbelief and asks X why he fights against Reploids. X flees, leaving Sigma to apparently perish.
Although defeated, Sigma's face reappears on a monitor after the credits roll, revealing that X only succeeded in destroying his body and that his "spirit" remains intact. Sigma states that he will find other bodies and threatens to return, a threat he follows up on in the next game.
A flashback in Mega Man X4 shows Sigma's past leading the Maverick Hunters. When Sigma learned that the berserk Zero had destroyed Garma's unit, he went to face the mysterious Maverick himself. He was overpowered by Zero in the ensuing battle, until Zero was suddenly immobilized by a great pain: the Maverick Virus taking hold.[2] Sigma took the chance to punch Zero's head and defeat him. He then ordered the Maverick Hunters to take Zero back for study, while he limped away for repairs. However, Sigma was unaware that he too was affected by the Maverick Virus, and in the following period of time it began influencing his thoughts towards rebellion against the humans.
Notes
↑The dates after November 22nd differ in the Japanese version of "The Journal of Dr. Cain", being on average a month or two later than the dates in the English version. Additionally, it says Sigma led the Hunters for two years with no issues, while the English version only indicates four months.
