Resident Evil 0 is a prequel to the events taking place in 1996's Resident Evil. Instead of playing as two separate characters in slightly divergent storylines, players now control two characters at once, switching between them at will to solve puzzles and to fight enemies. The female protagonist is Rebecca Chambers, member of the S.T.A.R.S. Bravo Team that would later be sent to Raccoon City to investigate a mysterious disappearance. A former Navy SEAL named Billy Coen, a convicted murderer awaiting his sentence, joins her on a train taking them to S.T.A.R.S. headquarters. Things take a turn for the worse when the train is suddenly attacked by zombies, thus beginning the fifth adventure in the horror series.<br><br>Players can examine their surroundings, pick up and combine items to solve puzzles or to use as tools, and fire weapons to stave off the advancing monsters and beasts. The partner system has players controlling one primary character and the computer controlling the other, or guiding both at the same time by simultaneously using the Control Stick and the yellow C Stick. Computer-controlled characters can be given the command to attack or to remain idle, conserving bullets when necessary. Acquired items can be swapped between characters when they are together in the same room, and partners can also split off to investigate certain situations alone, keeping the other informed using walkie-talkies.<br><br>Resident Evil 0 uses the same graphic engine introduced in the GameCube remake of Resident Evil. Players guide their characters across 3D environments while viewing the action from a series of fixed camera angles. Moving the character forward within the environment involves pushing up on either the Control Pad or Control Stick, while changing direction involves pushing left or right to rotate the character in place. Quick 180° turns can also be performed to escape harm's way. Saving the game once again relies on finding ink ribbons and then using them in a typewriter. Three difficulty levels include hard, normal, and easy, with optional vibration feedback for each setting.