Mega Man Maverick Hunter X

How to gain Zero's Buster:
If you pass over the X-Buster upgrade found in a Dr. Light capsule
during the game, you will be rewarded later in the Sigma Fortress
by gaining access to Zero's Buster, which is more powerful than the
normal X-Buster upgrade. To do this, simply do not enter the Dr. Light
capsule containing the X-Buster upgrade, and play the game all the
way to the third Sigma level. Upon facing Vile and beating him in
a battle, you will speak to Zero and he will give you his buster.
This buster shoots a different third level charge shot than Dr. Light's
upgrade, which is larger and more powerful.

Megaman X Anime Movie
Beat the game as X.

Play as Vile
Beat the game as X.

 