Agent Bill Sullivan (ビル・サリバン, Biru Sariban) is a federal agent who primarily handles incidents involving immortals.
Bill interrogates Ladd Russo in the aftermath of the Flying Pussyfoot incident, and is the one to inform him that Huey Laforet will likely be incarcerated in Alcatraz. As a result, Ladd deliberately tries to land himself in Alcatraz for the sole purpose of murdering Huey.
It is revealed in 1711 Whitesmile, that Bill has a granddaughter called Jessica Sullivan. She is a 'blonde and bespectacled' young woman by 2003, and like her grandfather works as an FBI agent for Victor Talbot.
Bill is thin, with droopy, half-lidded eyes that—coupled with his slow speech—give one the impression that he is perpetually on the verge of falling 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.