William Birkin had the distinct honor of being the youngest Umbrella scientist when he was hired at the age of sixteen. He came into Spencer’s lab at the same time as Albert Wesker, and was a brilliant scientist who made greater strides than any other scientist on the development of the T-Virus. He worked with Wesker on Lisa Trevor, trying to discover new viruses. During his research William uncovered a parasite that could be turned into a virus more powerful than the T-Virus, the G-Virus.

The G-virus became Birkin’s obsession in life, though he still managed to marry and have a child, and later transferred his work to a huge underground lab under Raccoon City. In July of 1998, Birkin learned there was an outbreak at the Spencer estate he used to work at. He and Wesker were shocked to see the long dead James Marcus was alive and the cause of the outbreak. That incident sparked a series of events that lead to Umbrella wanting a sample of the G-virus from William, who refused.

In September of that same year Umbrella operatives broke into Birkin’s lab, shot him and took samples of the viruses. With his last breath Birkin injected himself with the G-Virus, transforming himself into a giant armed monstrosity. Birkin killed the Umbrella operatives, which in turn broke the viles holding the samples of the virus. Rats then drank the samples and spread the virus across all of Raccoon City.

The monster that once was Birkin searched for his daughter because she was a genetic match, which is the only way the G-Virus larva could be transferred. Eventually, Leon and Claire destroyed the Birkin monster as they escaped on a train.