Zero

Zero (ゼロ) is one of the three main characters of the Mega Man X series, as well as the protagonist of the Mega Man Zero series and makes an appearance as Biometal Model Z in Mega Man ZX. In the X series, he is one of the highest ranking Maverick Hunters who is fighting to stop Sigma's forces of infected Reploids, known as Mavericks, from exterminating humanity.

Origin
Zero was created by the late Dr. Albert Wily somewhen in the year 20XX. Originally designed to deal with Mega Man and Bass, he was instead sealed away by Wily and later awakened by a group of Maverick Hunters prior to the first Mega Man X game. When he was first activated, he went rogue and destroyed an entire company of Maverick Hunters who dared entering Wily's lair. He was then challenged by the leader of the Maverick Hunters, Sigma. Even though Zero gained the upper hand during the battle, Sigma was able to defeat the red Maverick after taking serious damage and knock him out. He was then brought to Dr. Cain, who analyzed him.

The incident was covered up and Zero had no memories of fighting Sigma or being a Maverick while he was repaired (though his systems, like that of X, remained a mystery to the hunters' engeneering corps) and later became part of Sigma's group, the 17th Elite Unit.

Mega Man X
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Mega Man Zero
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Mega Man ZX
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Trivia

 * Zero was originally intended to be the main protagonist of Mega Man X, according to Keiji Inafune, Mega Man's creator. However, Zero was turned into a second character because Inafune feared Zero's look would not be approved by fans as a true Mega Man. He would eventually gain his own game series on the GameBoy Advance, Mega Man Zero.