Bill Sullivan (ビル・サリバン, Biru Sariban) is Edward Noah's coworker from the Bureau of Investigation and deals primarily with incidents involving immortals. He questioned Ladd Russo after the Rail Tracer events and was the one who told him that Huey Laforet would probably end up in Alcatraz. As a result, Ladd deliberately tried to get sent to Alcatraz, solely so he could track down Huey and kill him.
In 1711: Whitesmile, it is revealed that in Bill has a granddaughter called Jessica Sullivan. She is a 'blond and bespectacled' young woman in 2003, and like her grandfather works as an FBI inspector under Victor Talbot.