Sigma (Shiguma) is the primary antagonist of the Mega Man X series, considered to be the finest reploid of the time with circuitry designs meant to keep him from going Maverick. He was once the leader of the Maverick Hunters, but during a mission, he came into contact with Zero, who at the time was a Maverick. During their battle, Zero leaves Sigma badly damaged and infected with the Zero Virus. Although Sigma was initially stable after the incident, he soon became a Maverick and declared war on humans. He was defeated by X and Zero numerous times before being finally defeated on the moon in Mega Man X8, where they believed the Zero Virus will eventually die out. He is voiced by Mugihito from Mega Man X4 through Maverick Hunter X in Japanese, Charlie Fontana in the English adaptation of X4, Walter Roberts in the English adaptation of X7, Dave Pettitt in the English adaptation of X8, and Gerald Matthews in the English adaptation of Maverick Hunter X.

Sigma appears as a tall, bald humanoid in most of his forms. Although he has been destroyed countless times, Sigma's programming is able to survive the encounters in viral form, allowing him to build new bodies. It is implied that he grows more insane with each transformation, the most dramatic example being in X6, where, having deteriorated to a form that is barely coherent, screaming in broken sentences, and merely out for blood, only looks to destroy X and Zero directly. He manages to develop his data into the Sigma Virus, which allows him to easily infect numerous Reploids. He is fond of transforming himself into a larger and stronger form after his first normal form is defeated. He is a capable military leader, rendering him a devious and strategic opponent and allowing him to turn various Reploids, Mavericks, even other Maverick Hunters to his side, which is especially shown in X4, where he pushes the Repliforce and Maverick Hunters into nearly destroying each other. After his defeat in Mega Man X8, Lumine claims that Sigma is dead for good and will be unable to ever return, though it could be implied that the "Sigma" that was destroyed is actually a clone, and that Lumine is the real Sigma in disguise. Despite this claim, Sigma somehow manages to appear in the Mega Man/Sonic the Hedgehog crossover comic book arc, Worlds Unite, from Archie Comics (However, writer Ian Flynn claimed that the beginning of Worlds Unite takes place before Mega Man X8 and after Mega Man X7).