Billy along with Rebecca are played simultaneously and are the only playable characters in Resident Evil 0. Billy is differ from Rebecca in that he can not mix Herbs, however, he is not in need of health items as much because he has much more health. He also has a better chance at achieving headshots and can fire magnums faster as he doesn't flinch from the recoil like Rebecca. Puzzle wise, Billy is stronger and will be able to push heavier objects which is essential in some of them. Billy is also able to play the piano. When he joins Rebecca early in the game, Billy's inventory has six spaces and starts with a Handgun that's different from Rebecca's, Knife and the Lighter, his personal item, also required for some puzzles.
The number of Billy's alternate outfits available varies depending on the play version; In the original version, he have only one unlockable outfit while in HD Remaster, two more are included, being its based on characters from other Capcom titles. In general, the way all are unlocked also varies depending on the version:
Consists of an open dark blue suit, a yellow gold turtleneck shirt under a belt, black shoes and dark glasses.
In the original version, complete Main game on either Normal or Hard mode. Already in HD Remaster version, the outfit are available from the beginning.
Costume based on the outfits worn by the Wolf Force mercenary team in the 1990 arcade title, Mercs.[Excerpt 1]
Billy is a playable character alongside Rebecca in the extra mini-game Leech Hunter. He can collect only blue leeches. On Gamecube and Wii versions, players have the option to select Billy's outfits, being that he has same alternate costume seen in original game.
Resident Evil: The Umbrella Chronicles
Billy is a playable character along with Rebecca in the "Train Derailment" scenario. His gun is just called the "Handgun" and his melee attack is first pushing the zombie away then delivering a dropkick.
Resident Evil: The Deck Building Game
Billy is one of ten new character cards in the Resident Evil DBG expansion Alliance.
He has 80 Health. Billy's Level 1 effect allows him to gain one "Ammo x10" and gives him an additional Buy. His Level 2 effect allows him to move a card from his Discard Pile to his hand, but only if he has 10 or more cards in the pile. Interestingly enough, Billy's abilities go hand-in-hand with Rebeccas as the game progresses (of course they would both have to be present in Partner Mode). The "Ammo x10" cards that Billy gains can easily be trashed by Rebecca once they become less useful. And their mutual abilities to take cards away from the Discard Pile makes them a great combination if the player wishes to trim down his or her Inventory to the essentials. Of course this may be deliberate on the part of the game designers since Billy and Rebecca were the protagonists of Resident Evil 0. Billy's pose is his selection portrait in Umbrella Chronicles.
↑Excerpt from the News section of the Official HD Remaster website: This costume, based on the classic video game Mercs, suits Billy's wild personality perfectly. It looks especially cool when using heavy weapons.