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 Bassium, as Dr. Wily says to Bass that he created the Bassium 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 was found by Sigma and infected by him with one virus, turning Sigma in one Maverick. Zero now is one Maverick Hunter and is a good friend of Mega Man X.

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