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.