Mega Man X, more commonly known as just X, is the main protagonist of the Mega Man X series of games. He is a Maverick Hunter and Dr. Light's greatest creation, as well as the successor to the original Mega Man. He was the first robot to possess behavioral adaptation, making him capable of thinking, feeling and acting on his own, and is considered the "father" of the Reploids, as his general architecture was used to create the Reploids. X considers himself a pacifist at heart, wanting all of the fighting to be over, although he continues to fight to make that peace a reality.
Outside of the Mega Man X series, X has appeared is plenty of other games. He appears in the Mega Man Zero series of games, as a major part of the storylines, although he didn't physically appear in Mega Man Zero 4. Although it doesn't specifically state that it is X, one of the Biometals in Mega Man ZX is implied to be him. He appears as a cameo in many games, such as Tatsunoko vs. Capcom: Ultimate All-Stars, Dead Rising (as a costume), and Pocket Fighter. He is partnered with Zero in Project X Zone, as one of the Pair Units in the game.