Billy Coen was a lieutenant in the United States Marines, whose unit was dispatched to a remote region in Africa to intervene in a civil war going on there. Coen's unit became lost in the jungle en route to a suspected rebel camp. All but four of the Marines died on the journey, killed either by enemy fire or by the tropical heat. Lieutenant Coen and his commanding officer were among the survivors. By the time they finally reached the rebel camp, they were shocked to discover that it was an ordinary village. Coen's unit had been sent to the wrong place, based on faulty intel.
Coen's commander, however, was enraged and convinced the innocent villagers were involved with the enemy, and so he ordered his men to round them up and execute them. Coen attempted to intervene but was knocked unconscious. Later after it was proven that the villagers had been innocents after all, Coen was made the scapegoat and took the fall for his fellow soldiers. A year later, Coen, scheduled for execution, was being transported in a van by MPs Dickson and Elder through the Arklay Mountains when their vehicle was attacked by zombie dogs.
The MPs were killed, but Coen, riding in the back of the van, escaped after it overturned. On the run, he sought refuge aboard a nearby parked train, where he encountered rookie STARS officer Rebecca Chambers. Initially antagonistic towards one another, Coen convinced Chambers that they needed to cooperate in order to survive, especially once the train began moving and ultimately brought them to the abandoned Umbrella Corporation training facility.