William Birkin is the husband of Annette Birkin and father of Sherry Birkin. William is a genius that works for the Umbrella Corporation in its Bio-Weapons division. Along side fellow researcher Albert Wesker he was a part of the consipiracy to kill Umbrella Founder James Marcus. Marcus would be later found alive due to being exposed to his version of the T-Virus. When Wesker and Birkin discover that Marcus is behind the outbreak of the T-Virus in the Arklay Mountains they arrange to destroy the Umbrella Training Facility there and hopefully eliminate Marcus as well. Marcus is later killed by Rebecca Chambers and Billy Coen.
From Marcus' research on the Progenitor Virus that creates the T-Virus, WIlliam creates the G-Virus, a virus that is supposed to be superior to "T" in every way. During is work on "G" his confindence in his ability is shattered when news that Alexia Ashford's T-Veronica Virus is near compeltion and is expected to be the superior virus. Unfortunately for Umbrella, Alexia tests the virus on her self and declared dead (unknown to Umbrella she is merely in cryo-sleep for 15 years). This news is all William needs to regain his confidence and complete "G".
Resident Evil 2
Near the end of September 1998, Umbrella learns that William tends to take the G-Virus and give it to the United States Government. They send in an Umbrella Security Service Unit led by Hunk to retrieve all G-Virus samples. Birkin refuses to give away his life's work and is shot multiple times. The team take's all of the samples and leave Birkin to die. Unfortunately, Birkin was clutching a vial of the G-Virus and injects it into himself. He becomes a G-mutation and proceeds to eliminate the USS Team. During the battle (which only Hunk survives) the T-Virus infects several rats who then begin spreading the virus throughout the city. Within a matter of days the city is overrun by zombies. The police are unable to cope with the sheer volume and the survivors barricade themselves in the police station.
William is stalking the sewers and finds his way into the station (that is connected via the sewers) and infects Ben Bertolucci and Chief Brian Irons but they are not compatible matches so the infestation kills them upon gestation.
William encounters Claire Redfield and Leon S. Kennedy, his daughters self appointed protectors, while they are fleeing the police station through the sewer access. They do battle and William is defeated but the defeat causes him to mutate into his second G-form. William encounters Sherry, who had become separated from her protectors, while the girl is trying to escape through the sewers and successfully implants his daughter with an embryo.
Claire and Leon race to find the cure that they learn about from Sherry's mother Annette. Annette dies shortly after (depending on the scenario, either from a fight with William, or from internal injuries while trying to reacquire the G-Virus sample) but she provides the location of the antidote for Sherry and begs her daughters protectors to tell Sherry that despite her shortcomings as a mother that she has always loved her.
William attacks the fleeing individuals on the elevator in his second form but is once again driven off. He reappears while the duo race to give Sherry the antidote and battles them in his third and fourth forms. William is once again barring the way to Sherry and has now transformed into his fourth form. Once defeated he seems to melt into a puddle of ooze.
The trio (Sherry now cured of the G-embryo) are on the train thinking they are safe when William attacks a final time. His arrival on the train triggres a bio-hazard alarm and the train itself prepares to self destruct. Leon and Claire are successful in halting William until they can get clear of the the trains destruction. WIlliam is incinerated in the blast.
The G-Virus makes a reappearnce during the Hardaville Incident. An executive at WIllPharma has managed to acquire a sample of the G-Virus on the black market for the purposes of creating a vaccine for it. In reality he plans to sell it to third world warlords and make a fortune. He hires form TerraSave member Curtis Miller to cause a T-Virus outbreak at a local airport. Curtis does this and later sneaks into WillPharma to steal the G-Virus. Miller injects himself with the virus turning himself into a G-mutation. Curtis goes through two mutations before he is stopped by Leon S. Kennedy and Angela Miller.