Bill is 54 years old and has been a slave for most of his life. His decision to escape from his master came after accidentally killing their daughter. Within hours of the incident, Hannibal Hamlin found him, which saved Bill from being recaptured or falling prey to the Wasteland's dangers.
Bill has little in combat skill and has no military training. He's been relegated to cooking and cleaning due to this fact, although his culinary skills are awful at best. His other principal job is to take care of the Brahmin and the dog, Four Score. Bill has spent his whole life afraid and subservient and although he is free now, those habits are hard to break.
"Oh, ma'am, this place is heaven. Just a little bit of heaven here in the wasteland. Any slave makes it here, he's free. Just like that. And, Mister Hannibal, he's got a plan. A plan for heaven to be everywhere, not just here."
↑Fallout 3 Official Game Guide, page 43: "There is a band of abolitionist slaves deep in the Wasteland. They are all escaped slaves, seeking to end slavery for all time. You can learn more about them in Miscellaneous Quest: Head of State."