Sigma (Japanese: シグマ, Hepburn: Shiguma) is a fictional character and the main antagonist of the Mega Man X video game series.

Created by Dr. Cain, Sigma was considered the finest of the Reploids and was the first leader of the Maverick Hunters, peacekeeping androids who defend humans against their renegade counterparts.

[7] Though not a starring character, Sigma was referred to in an episode of the 1994 Mega Man cartoon, where X, Vile, and Spark Mandrill go back in time.

[8] He was created with the idea of being an "absolute evil" type character to fit the dark narrative of the games as Sigma is a villain despite being manufactured to be pure good.

[9] Starting with Mega Man X2, Inafune wanted to use a computer virus as a plot device, something he considered a more interesting idea than a tangible villain.

[10] Inafune denied multiple claims from his fans that Sigma was based on Sagat from the Street Fighter series and that he is instead an original character.

[12] With the creation of Mega Man X4, Inafune originally avoided featuring Sigma since the story of the such game involved a lack of black and white view of the scenario.

Peter Tieryas of Kotaku stated that he felt guilty after defeating Sigma, due to the Mavericks' origins as normal Reploids whose design was patterned after the series protagonist, and that they just wanted independence.

They called the Mega Man Sigma "a bit of a pest, to say the least," due to his ability to "leap into new robot bodies and other computer systems".

[25] Sigma's portrayal was the subject of analysis to Simão Cireneu Milani Addôr Nunes da Silva from Universidade Regional Integrada do Alto Uruguai e das Missões [pt] due to how Sigma's methods heavily contrasts with his underling X to the point that he does not share the same viewpoints from his ally Dr. Cain, who is interested in seeing X's entire potential.

What Sigma does not understand, and tries to prove that his creator is not right, is that, as a being that has a capacity that, in the commander's view, limits him as Maverick Hunter, it can be synonymous with unlimited potential.

[26] Artist Keisuke Mizuno was impressed by the character of Sigma upon first seeing his backstory to the point he would make headcanons about what is the true reason which made him betray the Maverick Hunters in the first game.

[27] With regards to Sigma's appearance in Marvel vs Capcom: Infinite, Mike Fahey from Kotaku thought the merging of Ultron and Sigma into an amalgamated being is "a great idea, and makes for a very cool visual", although he felt the game's idea of characters from Capcom's Mega Man, Resident Evil and Street Fighter franchises sharing the same world and knowing each other to be silly.

[28] Destructoid found Ultron Sigma's history too lazy as the player has to constantly face him in an appealing fight and then "fooders" in his place.