Series: Mega Man X
The Dr. Wily of the X series. Sigma is a Maverick Hunter gone bad -- really, really, really bad. He was behind the first Maverick rebellion and almost all the ones that followed (though usually from the shadows, not showing up in person till the end of each game).
Sigma is bald and has a vertical scar across each eye. He's very theatrical, practically born to be a villain. He likes scythes, claws, lightsabers, big guns, and BIG battle suits. His flair for multiple-round boss fights has led to the phrase "pulling a Sigma" for a defeated boss immediately coming back even stronger.
As of X8, if Lumine is to be believed, Sigma is dead for good. Don't make any large bets on it.