Voiced by David Webster (2002) and Steve Von Wormer (2007)

"Listen, little girl, if you haven't noticed there's some pretty freaked out things on this train. And I, for one, wanna' get out of here. I don't think we stand a chance of doing it alone."

A former Marine, Billy was wrongfully sentenced to execution for the slaughter of 23 people in Africa. However, when his transport through the Arklay mountains was overrun by mutated leeches (No, seriously...), he escaped. He proceeded to team up with Rebecca Chambers, and the two investigated zombie filled trains and a research facility in the woods. He was last seen in the woods around the mansion area, and hasn't been heard from since.

• Bad Ass
• Chained by Fashion: He wears handcuffs throughout the game, dangling off of one wrist. He manages to pick them off by the end of the game, and he discards them by tossing them off of a cliff.
• Death Faked For You: By Rebecca, at the end of the game.
• Hidden Depths: Despite how brutish he seems more often than not, he's remarkably good at playing the piano.
  • Tip of the iceberg. When Rebecca finds him after he gets washed away, they come upon a pile of skeletons. Somehow Billy is aware that Marcus used them as test subjects for the Mother Virus. Someone knows a little more than they're letting on.
• Huge Guy, Tiny Girl: One of the brawniest male characters, partnered up with the smallest adult female character.
• I Won't Say I'm Guilty: Granted, he's not.
• Inferred Holocaust: One possible reason he didn't show up in later games? The last you see of him is him walking into a forest infested with man-eating monsters, unarmed.
  • Then again, he can fit a grenade launcher in his pants.
  • Possibly averted, as Word Of God states that they plan to have him return in the future.
• Jade-Colored Glasses: Billy is almost bizarrely aloof about his conviction, resigned to his fate, and cynically shoots down Rebecca when she tries to be optimistic about it. And yet...
• Knight in Sour Armor: ... the cynical, wrongfully convicted murderer has a powerful sense of justice. He's the one who convinces Rebecca to partner up with him and he's the one who refuses to go back on his word.
• Semper Fi: An ex-Marine.
• The Nicknamer: To Rebecca.
• The Quiet One: Man of few words.
• What Happened to the Mouse?: Hasn't been seen or heard from since Zero. The only continuity nod is in Resident Evil 2, where Rebecca reported him dead.
  • Where she misspells his surname as "Koen"...

Voiced by Ian Downie (old) and Lyon Smith (young)

One of Umbrella's three founding members, he was murdered by Wesker and Birkin on Spencer's orders. He returns in Resident Evil Zero, where it's revealed that he is the one responisble for the Arklay Mountain and Mansion outbreaks.

• Achilles Heel: Sunlight.
• Back from the Dead: Thanks to some T-Virus infected leeches.
• The Bad Guy Wins: In a way: he dies, but his plan to bring down Umbrella goes off without a hitch.
• Big Bad: Of Resident Evil 0.
• Bishounen: As his younger self.
• Body Horror: Good god, Marcus.
• Corrupt Corporate Executive: Though he was more concerned with R&D than Spencer or Ashford.
• Doomed by Canon
• Evil Old Folks: Pre-murder.
• Evilutionary Biologist
• The Heavy: For RE0, overlapping with his status as Big Bad.
• Humanoid Abomination
• I Am Legion
• Mad Scientist
• Older Than They Look: His reborn human form resembles Marcus at about twenty.
• One-Winged Angel: His Leech Queen form.
• Revenge: His motivation is revenge on Spencer, Birkin, Wesker, and Umbrella as a whole.
• The Undead: Naturally.
• The Villain Makes the Plot: His actions after death lead to the original Resident Evil, and the entire series as a whole (which prompts Chris Redfield to leave for Europe after 6 months, Claire looking for him, Leon's involvement, etc)
• Voice of the Legion
• Weakened By The Light
• Weaksauce Weakness: Sunlight.
• Wicked Cultured: A talented opera singer.
• The Worm That Walks: Composed of thousands of leeches.