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In the DC comic book series, Birkin is seen slaughtering animals at the Raccoon Zoo before killing two employees in a violent rampage.
Oddly enough, Sherry mentions to Claire that her dad calls out for her, asking for help as he is attacked by "those monsters". However, in game, only his groans can be heard; he would call out for Sherry in Resident Evil 1.5 and the comics, though.
Also according to the leaked audio, he seemed to be nervous and aware that someone was after his research. Although one of the researchers knew what it was, he didn't touch the subject.
Birkin is the one who gave Wesker the experimental virus in order for him to fake his death in the Mansion Incident, causing him to be reborn and gain superhuman abilities. It is unknown if Birkin ever heard from Wesker after his resurrection.
Ada Wong obtained info that a researcher was close to the completion of the G-virus but refused to hand it over to Umbrella. Wesker knew that the researcher was William, and planned to save him and present the G-virus as a gift to The Organization. Their plan was foiled by the U.S.S. who stole the virus for Umbrella and killed Birkin.
William Birkin was originally to have a much bigger role in Resident Evil Outbreak in his G-1 stage, appearing in an Umbrella facility. This was shown in the Resident Evil Online trailer. However, when the production had to be scaled down, he was ultimately removed in all but the opening cutscene, though his in-game model was kept.
When Brian Irons confronts Claire in his taxidermy room and divulges information about the G-Virus while threatening her, he implies that Birkin had a hand in the development of the T-00 (referred to as the monster tearing the precinct apart) due to it being a product of the G-Virus. This was actually an error in the localization. In the original game, Irons simply explained that it rapidly induced human evolution, without even mentioning the T-00 at all.