Mega Man X3

But something goes terribly wrong, and now Mavericks have begun to appear everywhere! It's up to X and Zero to find out who is behind this Maverick revolt and bring them down for good! For the first time in the series, players can play use X's friend, Zero.

However, he is pretty much restricted to only about one third of each level, and has only one life (i.e when he loses all his energy, he is put out of the fight for the rest of the game).