Zero

Zero was made by Dr. Wily in the year 20XX. Originally created to destroy Mega Man, 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.

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.