The deuteragonist of the Mega Man X series and star of the Mega Man Zero series. He is a red robot that was created by Dr. Wily towards the end of the Wily Wars as a robot that could easily defeat Mega Man, Bass, and any other creation. He was developed at the same time that Dr. Light was developing Mega Man X. He was finished before X, but due to hsi violent and disobedient temperament he was sealed away, forgotten about when Wily died. He was revived by a group of maverick robots in 21XX, and joined them in their attacks against humanity. As one of the most dangerous mavericks around, he soon became the focus of the Maverick Hunters leader, Sigma, leading to a battle between them. During the battle, Zero's inbuilt maverick virus, installed by Dr. Wily on creation, was released. This led to Zero becoming more rational and focused on justice, but had the opposite effect on Sigma, turning him Maverick.

At the start of the first Mega Man X game, Sigma has outed himself as a maverick and so Zero has taken over leadership of the Maverick Hunters, with X as his protege.