Resident Evil Zero

Note: This game is also titled Biohazard 0.
# Alternate costumes

Successfully complete the game with any rank on 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 to 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. Note: To get a rank after completing the game it must be played on the normal difficulty setting. You will not get ranked on the easy difficulty setting, but can still unlock the Leech Hunter mini-game.
# Leech Hunter mini-game bonuses

Successfully complete the Leech Hunter mini-game with one of the following ranks to unlock the corresponding bonus:

"A" rank: Infinite ammunition for any weapon
"B" rank: Magnum
"C" rank: Hunting Gun
"D" rank: Handgun
"E" rank: Submachine Gun

# 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.
# Extra inventory slot

When playing as Billy with Rebecca as the alternate character, have Billy hold all the pistol ammunition for both characters and keep her gun fully loaded. This gives you one free inventory slot.
# Pause timed puzzles

Change to your partner during a timed puzzle. Remain idle at the error message to give yourself more time to study the puzzle.
# Piano room

When you get to the piano room, make sure you have both characters with you. Have Billy play piano. A door will open by the bar. Have Rebecca go in and take the battery. The door will close. Have Billy play piano again and the door will open.
# Better shotgun use

Once you get the shotgun, do not use it. Instead, leave it in the room with the scale you put the two figures on. Find, but do not pick up, lots of shotgun ammunition. After you save her from the pit, give all the hand gun ammunition to Billy and collect the shotgun ammunition. If you get it all, she will have about 60 rounds, which is much better than using two hand guns.
# Dail door number puzzle

Enter "4863" on Disc 2, where the cable car is located.
# Brake system puzzle

The main code is 9 + 9 + 9 + 9 + 9 + 9 + 9 + 9 + 7 + 2 = 81. This should activate the brakes for the train.
# Fountain circle puzzle

When you get to the fountain circle with the six animal statues on Disc 1, use Billy to light the statues in the following order: Deer, Wolf, Horse, Lion, Snake, Eagle.
# Collecting Leeches in Leech Hunter mini-game

While playing the Leech Hunter mini-game, kill everything in the Mansion, Train Tunnel, and Basement before picking up any of the leeches. Once you pick one up, you cannot put it down. Each player can find a maximum number of 50 leeches (100 total leaches in the game). One item slot can only hold 10 leeches only -- be careful.
# Determining weapon in Leech Hunter mini-game

There is a way to determine if you will get the submachine gun or the magnum. Look at the two herbs in front of the stairs in front of you at the start of the mini-game. If the colors are blue and green, you will get the magnum. If the colors are green and green, you will get the submachine gun. The submachine gun can do what the magnum can and requires a lot less ammunition. Green and green is highly recommended. If you want the magnum on the Leech Hunter mini-game, load your game on Disc 1 (only if you have passed the game once and have unlocked the Leech Hunter mini-game), then play the mini-game. You will see a Green Herb and a Blue Herb, indicating that the Magnum is there. If you want the Submachine gun, load your saved file with mini-game on Disc 2 and play the mini-game. You will see two Green Herbs, indicating that the Submachine gun is in the game.