William "Billy" Coen was a Second Lieutenant in the United States Marine Corps.who, along with a team of Marines, were sent into hostile territory in Africa to attack a village used as a terrorist training ground.
Unfortunately, the air drop was off-target, and as such the team had to march across miles of hostile terrain to their target objective. The heat, combined with enemy fire, soon reduced the team to four, and when they finally arrived, they found the village was actually full of innocent civilians. Despite this, the CO ordered them to shoot. It is implied that Billy never took part in this, but he neither confirms nor denies taking part in it to Rebecca, which claimed 23 lives in total, and upon returning to base their superior and base commander, Samuel Regan, had Billy tried, court-martialled, and sentenced to death.
En route to the base for his execution, the military transport vehicle used to take him there was forced off the road by James Marcus, and then Cerberus attacked and mauled the MP's guarding Billy, causing him to run for his life. Billy then sought refuge in the Ecliptic Express, a stranded train, where he meets Rebecca Chambers. Despite their radically different backgrounds, the two form an alliance in order to secure their survival. The two use their talents to pass through many of the obstacles they face, and discover the Umbrella Corporationís secrets.
Both characters are monitored by James Marcus, who sends various creatures to subdue the two. Billy proves his loyalty by saving Chambers several times during the game. Coen and Chambers eventually meet Marcus towards the latter portion of the game. After revealing the details of his mortal demise, Marcus mutates into a Queen Leech.
The two manage to temporarily stop the creature, only to discover that a self-destruct protocol has just been activated. Matters become even more complicated when the leech returns. In the final battle sequence, Chambers exposes the leech to sunlight, thus making it vulnerable to attack. Coen proceeds to deliver a final shot, which instantly destroys the leech. In wake of the destruction, Chambers allows Coen to escape, by telling her authorities that Coen subsequently perished in the Arklay Mountains after their encounter, keeping his dog-tags as proof. The two bid farewell, and Billy disappears soon after, not seen or mentioned ever again.