Agent Bill Sullivan (ビル・サリバン, Biru Sariban) is a federal agent working for the Bureau of Investigation and deals primarily with incidents involving immortals. He questions Ladd Russo after the Rail Tracer events and tells him that Huey Laforet will probably end up in Alcatraz. As a result, Ladd deliberately tries to get sent to Alcatraz, solely so he can track down Huey and kill him.
In 1711: Whitesmile, it is revealed that Bill has a granddaughter called Jessica Sullivan. She is a 'blonde and bespectacled' young woman in 2003, and like her grandfather works as an FBI inspector under Victor Talbot.
Bill is thin, with droopy, half-lidded eyes that - coupled with his slow speech - give the impression that he is always on the verge of going to sleep. He has defined cheekbones and combed back brown hair in the novels and 2015 manga, whereas he is tall and blond in the 2007 anime in a design reminiscent of Donald's novel illustration.