At the young age of 16, William Birkin was chosen alongside future ally Albert Wesker to serve the pharmaceutical corporation; Umbrella, in the management training facility deep within the forest of the Arklay Mountains, until they were both then chosen to serve Umbrella within the Arklay Research Facility. Developing a fascination for the Ebola virus discovered in Africa, Birkin experimented with it while Umbrella officially stated to the public they were searching for a cure. It was then that both Birkin and Wesker were introduced to their newest test subject: Lisa Trevor, daughter of the very man that helped design the famous Spencer Estate within the Arklay forest.
Soon, his work would be interrupted by the sudden rumor of a young prodigy of the Ashford brood, a young girl known as Alexia Ashford. As time went on, Birkin would come to despise the younger researcher and hold a grudge that would stick until her apparent death in 1983. Despite testing numerous viral strains upon Lisa Trevor, including the infamous Ebola virus, Birkin found success in his creation of the B.O.W.'s known as the "Hunters," in 1981.
Eventually, both Birkin and Wesker would be promoted within the Umbrella ranks, but not without blood being spilled. Their former mentor, James Marcus, was conspired against, and eventually assassinated with both Birkin and Wesker overlooking. The last words James Marcus would hear would be William Birkin's taunting "I'm going to take over your research!" Indeed, although James Marcus had created the T-Virus strain, official credit was given to William Birkin following the untimely death of James Marcus.
Experimentation continued upon the guinea pig that Lisa Trevor had become, including injecting her with an NE-α Parasite, as well as an alternate strain of the Progenitor Virus. This would soon become the unstable viral strain known as the G-Virus. Much different from the T-Virus, the G-Virus would mutate the host in a series of plateaus, that which were deadly, yet unpredictable. Ironically, Umbrella began to conspire against William Birkin, wanting the G-Virus for themselves, they sent a squad led by HUNK to retrieve a sample, yet when they arrived, in the heat of the moment they gunned Birkin down.
Whilst dying, his wife Annette frantically went to try and get help. Meanwhile, Birkin did the only thing he thought plausible in the situation before his imminent death, injecting himself with a sample of the G-Virus. This would act as a catalyst for the events that would soon lead to the infection of Raccoon City itself.
Part of the G-Virus side effect was that the virus itself sought out genetic matches, such as family, and sought to impregnate them with the virus in an act of viral incest. Throughout his stages, William sought out his daughter Sherry Birkin, the human side of him yearning to find his daughter, while the monster side of him wanting to impregnate a genetic match. Managing to hunt Sherry down, he impregnated her before the heroes could stop him. Afterwards, Birkin, now known as G (for the virus), hunted down and killed his wife Annette. It took the efforts of Leon S. Kennedy to cure Sherry of the G-infection. G was later destroyed when a train it had latched onto self-destructed.
William Birkin would go through 5 stages of mutation while infected with the G-Virus.
Perhaps the closest in appearance to his human form, damaged tissue is replaced with unnatural muscle mass, while the trademark eyeball forms upon the left shoulder. His strength increases vastly, while using a pipe he found earlier as a melee weapon.
Appearing less and less like his human self, the left arm begins turning into a claw, while his human head is forced down into his torso with a new skull replacing the old human head.
Perhaps the most demonic-looking of his transformations, Berkin now sports a mouth filled with syringe-esque teeth, while also now having four arms with deadly claws on all four appendages. His strength increases further, as well as his acrobatic skills. This is the last humanoid version of his transformation, with Berkin becoming known as simply G.
Now a hexapedal creature with razor sharp protrusions coming from its face, G's speed increases as well as its agility. It is often considered to move like a dog or a wolf.
The most bizarre of G's transformations, and the last, this creature hangs to the back of the train Leon and company are upon as they attempt to escape. The train is eventually blown up with G still attached. Later on, Albert Wesker came to pick up the pieces for a sample of the G-Virus.