William Birkin was a former co worker of Albert Wesker. He is also the creator of the G-Virus and is partially responsible for the outbreak which caused Raccoon City's destruction.
William Birkin is the husband of Annette Birkin and father of Sherry Birkin. William is a genius and works as a scientist for the Umbrella Corporation. Birkin along with Albert Wesker was ordered to assassinate James Marcus. Marcus is later found alive due to his version of the T-Virus. Wesker and Birkin soon find out that Marcus was alive and was the cause of the T-Virus out break in the Arklay Mountains. With this knowledge Birkin and Wesker planned on destroying the Umbrella Training Facility in order to eliminate Marcus. Marcus ended up being killed by Rebecca Chambers and Billy Coen.
Using the research on the Progenitor Virus and the T-Virus, Birkin created the G-Virus, a virus in which was supposed to be superior to the T-Virus. During his work he finds out that Alexia Ashford's T-Veronica Virus was nearly complete and was supposed to be the superior virus. Alexia tested the virus on herself and was declared dead by Umbrella. Unbeknownst to them Alexia was in a cryo-sleep for fifteen years. When receiving this news Birkin continues his work on the G-Virus.
Resident Evil 2
In September 1998, Umbrella found out that Birkin was planning on giving the G-Virus to the United States Government. Umbrella sends out their Secret Service Unit (USS) led by H.U.N.K to retrieve all samples of the G-Virus. When the team arrives Birkin refuses to give up his life's work and is shot multiple times. The team retrieves all the samples and leaves William to die. Unbeknownst to them, William Birkin was clutching a sample vial and in injects it into himself. Birkin then becomes a G-Mutation and begins eliminating the USS Team (Only H.U.N.K manages to survive). During the battle however, the T-Virus infects some rats who in turn spreads the T-Virus around the city. Before you know it the city is infested with zombies. Remaining survivors began to seek refuge at the Police Department.
Birkin travels around the sewers and finds his way into the underground station. He infects Ben Bertolucci and Chief Brian Irons. Due to the fact the two did not have compatible DNA they both die shortly after being infected.
William Birkin soon runs into Leon S. Kennedy and Claire Redfield, along with his daughter Sherry while they are fleeing from the Police Station. not fully aware of what he is doing he attacks them. They manage to defeat William but this causes him to mutate to his second G-Form. William does find Sherry after she is separated from Claire and Leon. At this time Birkin implants an embryo inside his daughter.
Claire and Leon soon finds out from Annette Birkin that there is a cure to the virus. Annette dies shortly afterwards either by William or internal injuries she receives while trying to retrieve a sample of the G-Virus. Before she dies however, she informs Claire and leon of the location of the anecdote and to tell Sherry that despite her shortcomings as a mother, she always loved her.
William attacks the duo once again in his second form. After being defeated for a second time, he appears again while the two race to get the anecdote for Sherry. He battles them in his third and fourth forms during this time. Trying to make his way to Sherry in his fourth form he is defeated again but seems to melt into a puddle of ooze.
Now that Sherry is cured, the trio uses the underground train to escape the facility and reach safety. Unfortunately, Birkin appears once again and attacks in his final form. Upon his arrival, a bio-hazard alarm goes off which triggers the train's self-destruct mechanism. The team manages to avoid dying and get's clear of the trains blast. William Birkin on the other hand is incinerated within the train.