- Resident EvilIntroducing
- Resident Evil 2Introducing
- Resident Evil 3: NemesisIntroducing
- Resident Evil Gun Survivor Introducing
- Resident Evil – Code: VeronicaIntroducing
- Resident Evil OutbreakIntroducing
- Resident Evil 4Introducing
- Resident Evil: The Umbrella ChroniclesIntroducing
- Resident Evil: DegenerationIntroducing
- Resident Evil 5Introducing
- Resident Evil: The Darkside ChroniclesIntroducing
- Resident Evil: RevelationsIntroducing
- Resident Evil: Operation Raccoon CityIntroducing
- Resident Evil 6Introducing
- Resident Evil: DamnationIntroducing
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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."
- 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.
- 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.
- Tattooed Crook: Averted. He's the only convict among the series protagonists and has a very prominent tattoo, but Billy is a decent guy and not actually a criminal, and closer examination reveals that his tattoo reads, "Mother Love."
- 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"...
Dr. James Marcus
Voiced by Ian Downie
One of Umbrella's three founding members along with Ozwell E. Spencer and Edward Ashford, the one who figured out how to make the Proginetor Virus into the T-Virus. He was murdered by Wesker and Birkin on Spencer's orders.
- Asshole Victim: See Corrupt Corporate Executive and Mad Scientist.
- Bishounen: His younger self was, apparently.
- Corrupt Corporate Executive: Though he was more concerned with R&D than Spencer or Ashford.
- Doomed by Canon: Since he's a Posthumous Character that's not seen in other Resident Evil games.
- Evil Old Folks: Like all the original founders of Umbrella (except Ashford).
- Evilutionary Biologist
- Mad Scientist: Like almost everyone involved with Umbrella. The medieval torture dungeons found in the training facility speak for themselves.
- Wicked Cultured: A talented opera singer.
Mysterious Young Man/Reborn Dr. Marcus/The Queen Leech
Voiced by Ian Downie (old) and Lyon Smith (young)
A young man in strange clothes that causes the mutated leech outbreak on the Ecliptic Express. He claims to be Dr. Marcus reborn and wants to destroy Umbrella. He's actually Dr. Marcus' greatest creation the Queen Leech, which absorbed Dr. Marcus' body and memory.
- Achilles' Heel: Sunlight.
- Back from the Dead: Thanks to some T-Virus infected leeches.
- Big Bad: Of Resident Evil 0.
- Body Horror: Good god, Marcus.
- The Heavy: For RE0, overlapping with his status as Big Bad.
- Humanoid Abomination
- I Am Legion
- One-Winged Angel: His Leech Queen form.
- Predecessor Villain: Could arguably be considered this for the entire series, since his actions lead directly to the Mansion and Raccoon City incidents.
- Revenge: His motivation is revenge on Spencer, Birkin, Wesker, and Umbrella as a whole.
- The Undead: Naturally.
- Voice of the Legion
- Weakened by the Light
- Weaksauce Weakness: Sunlight.
- The Worm That Walks: Composed of thousands of leeches.