Z-Prototype, or Zero-Prototype, or you can call him Zero. Is a robot with very advanced technology and free will. He mostly uses martial art attacks like an uppercut or a punch. He can deflect any bullet (as seen when "fighting" him in Mega Man Unlimited) whether it'd be a rocket or a powerful explosion. Not even Mega Man's charged shot could dent him. He was created by Dr. Wily. 
He doesn't talk alot (unless you want him to.) 
if you do nothing, he'll just look at you. He just has no purpose.