Resident Evil Zero, known in Japan as Biohazard is a survival horror game that was developed and published by Capcom for the Nintendo GameCube and released in 2002. It is the fifth game in the main Resident Evil series by release order (not counting the GameCube version of the original Resident Evil as a separate game) and was the last of the main titles to use the original Resident Evil gameplay system prior to the release of Resident Evil 4. The game's storyline serves as a prequel to the first Resident Evil, covering Rebecca Chambers' ordeal a day prior to the Mansion incident. The main gameplay feature of Resident Evil Zero, dubbed the "partner zapping" system, is unique in the series. So, instead of choosing a single character to play through the whole game, the player controls the two main characters, Rebecca Chambers (S.T.A.R.S. Bravo Team's medic, seen in the original Resident Evil) and Billy Coen (an escaped convict) simultaneously through the entire game. Both characters can travel together, with one character being controlled by the player and other by the computer, or split up and search separate areas each at the same time. This dual control is central to solving some of the puzzles in the game.

The difference between both characters is that Rebecca can acquire chemicals with her mixing kit and can get into tight spaces, but is more fragile to enemy attacks. In contrast, Billy can handle heavy objects, has an oil lighter and can sustain more damage than Rebecca, but cannot mix herbs (in contrast from previous games, in which any character could mix herbs).

Unlockables

Unlock New Costumes
If you complete the game on Normal or Hard mode, you'll find the Closet Key in Rebecca's Inventory when you start a new game.

Billy will get a suit and Rebecca will get a leather outfit or a cowgirl uniform.

Easter Eggs

Unlock Leech Hunter Game
Beat the game on Easy, Normal, or Hard mode to unlock this mini-game.

Secrets

Leech Hunter Rewards
100 leeches: All weapons found have unlimited ammo.

90-99: Magnum revolver in room 202 of the train.

60-89: Unlimited ammo with the hunting gun.

30-59: Equipped handguns have unlimited ammo.

1-29: Sub-machinegun Bullets added to Rebecca's inventory.

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 on
normal or hard mode.

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

Here's how you defeat the first boss in the game, using only the knife: