Billy Coen is the co-protagonist of Resident Evil Zero along with Rebecca Chambers.
He is first mentioned in a police report filed by Chambers that can be found in the Nintendo 64 version of Resident Evil 2, foreshadowing the events of the prequel. A former Second Lieutenant in the United States Marine Corps Force Reconnaissance, Billy is sentenced to death by a military court for massacring 23 people in murky circumstances during a mission in Africa. Billy's flashbacks imply he is innocent and the victim of a cover-up. He escapes en route to his execution and seeks refuge in the Ecliptic Express, a stranded train, where he meets Rebecca Chambers. The two form an alliance in order to survive, using their talents to discover the Umbrella Corporation's secrets.
Both characters are monitored by the Queen Leech-James Marcus hybrid, who sends various creatures to subdue them. After revealing the details of his demise, the hybrid Marcus fully mutates into the Queen Leech. In the final battle sequence, Billy and Rebecca destroy the Queen Leech by exploiting its vulnerability to sunlight. Rebecca allows Billy to escape, believing his claims that he is innocent and telling the authorities that he subsequently perished in the Arklay Mountains after their encounter. Billy also appears in The Umbrella Chronicles.