A world-renowned scientist working for Umbrella, William Birkin is the primary figure responsible for the development of the G-virus. He was also a core component in continuing research on the T-virus where Marcus left off. However, some unpleasantness arose between him and Umbrella, prompting him to approach the U.S. military. Umbrella responded by sending in their special forces.

Shortly before his death at their hands, he injected himself with the G-virus and transformed into a monster. He is survived by a daughter, Sherry, from his late wife, Annette.