Husband of Annette and father of Sherry, William Birkin was the brilliant scientist who developed the G-Virus for Umbrella, Inc. in the underground laboratories beneath Raccoon City. Fearful that Umbrella would be as careless with the G-Virus as they had been with the T-Virus, Birkin opted not to turn his life's work over to the company.
This didn't sit well with Umbrella, who sent a squad of Cleaners to assassinate Birkin and steal a sample of the G-Virus. The mission did not go entirely as planned. The Cleaners did shoot Birkin, but they only wounded him, and they also grabbed the wrong sample case (they stole a case of T-Virus samples instead of G). The dying Birkin injected himself with one of only four known samples of the G-Virus, mutating into an ever-evolving G-Type creature, a hideous mockery of humanity.
In this form, he hunted down and killed all of the Cleaners save Hunk, and in the process the case of T-Virus broke open, and the virus was spread by rats through the city. The mutated Birkin then spent the rest of his unnatural life skulking around in the sewers of Raccoon, occasionally turning up to menace Leon S. Kennedy and Claire Redfield, and he was also responsible for the deaths of Ben Bertolucci and Brian Irons.
After evolving into numerous forms, each more powerful than the last and looking less and less human with each transformation, Birkin was finally killed aboard an Umbrella train when it was blown up by Leon and Claire following their escape from Raccoon City.