William Birkin is one of the main antagonists in the Resident Evil series, specifically Resident Evil 2. After completing his G-virus project (Golgotha virus), he is mortally wounded by agents of the Umbrella Special Forces and his work is stolen. Left for dead, he injects himself with the G-virus, thus turning himself into a monster known as "G". He then kills his attackers, causing a T-virus vial to be leaked into the city's drainage system, resulting in the zombie outbreak in Raccoon City. During the course of the game, he encounters the main characters, Claire Redfield and Leon Kennedy, as he undergoes gradual mutation and begins losing any resemblance of his original human form. He is eventually defeated by Claire and Leon, with the help of his daughter Sherry. Birkin is mentioned in Wesker's Report as the scientist who helped Wesker fake his death and is retroactively mentioned in the GameCube remake of the first Resident Evil. He also makes an appearance in Resident Evil Zero, where he assists Wesker from behind the scenes, and also appears in a cut-scene in The Umbrella Chronicles along with Albert Wesker.
In the first Resident Evil film, Birkin makes an uncredited cameo appearance as the head of the Nemesis project, who was also the film's narrator. According to an audio commentary by Paul W. S. Anderson, Isaacs was planned to reprise this role in the sequel Resident Evil: Apocalypse, but left the project for undisclosed reasons, so an original character named Dr. Alexander Isaacs (played by Iain Glen) was created to fulfill Birkin's role in the sequel.