Zero

Zero was made by Dr. Wily in the year 20XX. Originally created to destroy Mega Man and Bass, he was instead sealed away for a century when he woke up and killed several Maverick Hunters. He then fought Sigma and transferred the Maverick Virus to him during the fight. This would begin the course of events which instigated the Maverick Wars and would create the Sigma Virus. You were able to call him in Mega Man X3 to play as him. Ever since, he has been playable.

It was not comfirmed by Capcom that Zero was Dr. Wily's last creation, but there are inumerous facts that indicates it. The most notable is Bass' ending in Mega Man 2: The Power Fighters, where Dr. Wily shows one blueprint with a robot similar to Zero, saing that this would be his best creation. Its possible that Zero is made of Bassnium, as Dr. Wily says to Bass that he created the Bassnium and Bass in one accident, and he could improve it.

There is a popular fan theory that Bass stole the plans for Mega Man X when he raided Dr. Light's lab in Mega Man 7 and that Zero was based off of these plans. However this has not been confirmed by Capcom.

In 21XX, Zero reappeared, as a Maverick. Sigma was dispatched to defeat him, but was near defeated by Zero - however, Sigma managed to smash the crystal on Zero's forehead. This rendered Zero unconscious but contaminated Sigma with the Maverick Virus. Zero was repaired and now is a Maverick Hunter and is a good friend of Mega Man X. Zero has dedicated his life to pacifism, and is known as a great hero alongside Mega Man X.

Many years after the Mega Man X series, he was ressurected by Ciel using a Cyber-elf called Passy.

Trivia

 * Zero was originally intended to be the star of Mega Man X, according to Keiji Inafune, Mega Man's creator.
 * Early designs of Zero indicated that he was originally meant to be a girl.
 * His favorite color is widely believed to be black. This may be a nod to the Black Zero in Mega Man X2, and the hidden Black Zero codes in Mega Man X4, Mega Man X5, Mega Man X6, and Mega Man X8 (yes, there is a secret Black Zero in Mega Man X8).  Black Zero is also the Hyper Mode of Zero in Mega Man X: Command Mission.