Billy Coen is an ex-marine convicted of a crime. He is one of the main characters in the Resident Evil series prequel, Resident Evil 0.
(Key note everyone: Billy Coen was mentioned in one of Rebecca Chambers files in the Nintendo 64 version of Resident Evil 2. Of course Resident Evil 2 came out before the prequel, Resident Evil 0. It wasn't until Resident Evil 0 came out for the Nintendo Gamecube that Resident Evil fans were introduced to Billy Coen.)
Resident Evil 0
Billy Coen was an ex- marine who was being transferred by the Military Police for execution. Unfortunately, the vehicle transporting him was attacked by mutated canines. With the guards dead, Coen manages to partially free himself from the cuffs on his wrists while retrieving a gun from one of the dead officers. Coen then takes refuge in the Ecliptic Express train where he runs into S.T.A.R.S. Bravo Team Medic, Rebecca Chambers. After saving Rebecca from zombies the two plan to work together in order to survive. Rebecca Chambers secretly plans on turning Billy in once she manages to reach safety. Unfortunately for her, plans change when the train starts moving and crashes into an Umbrella Research Facility. Rebecca and Billy continues to work together to find out what is going on. The two soon finds out that they are being monitored by Dr. James Marcus.
After working together curiosity reaches it's peek and Rebecca asks Billy why was he being sentenced to execution. Billy informed her that he was an ex-marine and while on a mission he was convicted of killing 23 villagers, but was framed by his platoon.
Soon after his confession, the two are confronted by James Marcus. At this time the T-Virus began to take over his body, turning Marcus into a Queen Leech. Billy and Rebecca then work together as a team to take down Marcus and escape the facility. Once they manage to escape the underground facility, Rebecca unlocks the handcuffs attached to one of Billy's wrists and collects his dog tags. She tells him that in her report she will announce his death and use the dog tags as proof. They part ways, with Billy entering the forest and Rebecca heading for the Spencer Mansion to regroup with her fellow teammates.