Resident Evil 0
Billy Coen is a former marine that was being transferred by the Military Police to be executed when the convoy is attacked by Cerberus hounds. With his guards dead, Billy (partially) frees himself from his restraints, removes one of their firearms and flees from the hounds. Billy takes refuge in the stranded Ecliptic Express train where he runs into Rebecca Chambers. After saving her from Zombies, the duo agree to an alliance in order to survive the zombie hords. Rebecca then plans to arrest Billy and take him in, until plans change when the train starts up and crashes into an Umbrella Research Facility. From there Rebecca and Billy continue to work together to find out what has been going on, until they find out they're being monitored by Dr. James Marcus.
After running through the facility and and saving Rebecca, Billy tells her that he was convicted for killing the 23 villagers on a mission, but he was framed for the murders by his platoon.
When the pair are confronted by Doctor Marcus he transforms into a gigantic Queen Leech. Billy and Rebecca then use their team work to take down Marcus and escape. Upon escaping the underground facility Rebecca finally takes the handcuff that's locked on to Billy's wrist off, and collect his dog tags. Rebecca informs him that her report will say that Lieutenant Billy Coen was killed in and take the tags back as proof. After saluting him the pair part ways. Billy walks into the forest to never be heard from again, and Rebecca makes her way to the safety of the Spencer Mansion.
Billy Coen was first mentioned in a file by Rebecca Chambers in the Nintendo 64 version of Resident Evil 2, to foreshadow the events of the planned prequel for the Nintendo 64. Resident Evil Zero was pushed back to the Nintendo Gamecube, then finally Resident Evil fans were introduced to convicted murderer, Billy Coen.