Billy Coen was a U.S. Marine Corps Second Lieutenant-turned death-row inmate that escaped into the Arklay Mountains after the military transport vehicle escorting him was forced off the road by a pack of Cerberus.
He took refuge in the nearby Ecliptic Express where he would later meet Rebecca Chambers, who had arrived with S.T.A.R.S. to investigate reports of murders in the vicinity in the events of Resident Evil 0.
During the game, it was revealed that his commanding officer ordered the execution of innocent civilians in a small village suspected of being a training camp for guerillas. Because innocent civilians were slaughtered, Billy's commanding officer framed him for the attack which is why Billy was court martialed.
Billy killed the Queen Leech after Rebecca leaves it vulnerable to sunlight. Rebecca then takes Billy's dog chains and says that he is "officially dead" allowing him to escape. His location is still unknown.