William Birkin is introduced in Resident Evil 2. The first thing you see about him is his name mentioned in files. Later in the game, you discover that he is the mind behind the G-Virus and that he has injected the virus into his body after receiving injuries from Umbrella mercenaries.
 Plot Contributions
William appears many times throughout the game as a boss fight. He also infects Sherry Birkin, his daughter, injures Ada Wong, and, in the Claire Redfield scenario, kills his wife Annette Birkin. He goes through many stages in his grotesque mutation, and is eventually destroyed by the self-destruct feature of a train that the characters use to escape from Raccoon City.
 Personal History
Not much is specified about Birkin's past, apart from the fact that he joined Umbrella about the same time as Albert Wesker. It can be assumed that Birkin had a rivalry with Alexia Ashford, due to the fact that it is mentioned in a file in Resident Evil Remake.