Resident Evil 0 [GC]

Alternate costumes:
Successfully complete the game with any rank under the normal or hard difficulty settings. Start a new game. A Closet Key will appear in your inventory. Use it to unlock the closet in the room where the Hunting Gun was first found to access new costumes for Rebecca and Billy.

Completion bonuses:
Successfully complete the game with a "B", "C", or "D" rank and unlock the Leech Hunter mini-game.

Successfully complete the game with a time between 3:31 and 5:00 for an "A" rank to unlock the Submachine Gun and Leech Hunter mini-game.

Successfully complete the game with a time of 3:30 or faster for an "S" rank to unlock the Rocket Launcher, Submachine gun, and Leech Hunter mini-game.

One hit kills:
When playing as either Rebecca or Billy and you get bit by a zombie, move the character opposite of your main character directly behind the one you are controlling to have him or her shoot the zombie in the head and kill it in one hit, while the zombie deals you no damage. Note: You must have pistol equipped to save ammunition for a one bullet kill.

Unlock Ending Bonuses
You can get certain prizes depending on your gameplay rank, which is how fast you beat the game.

Here are the ranks and prizes.

D:(9:01 or more) Leech Hunter and Closet Key
C:(7:01 - 9:00) Leech Hunter and Closet Key
B:(5:01 - 7:00) Leech Hunter and Closet Key
A:(3:31 - 5:00) Submachine Gun, Leech Hunter, Closet Key
S:(3:30 or less) Rocket Launcher, Submachine Gun, Leech Hunter, Closet Key