Billy Creel is a resident of Megaton in Fallout 3. He can often be found in Moriarty's Saloon drinking Nuka-Cola. He lives with his adopted daughter Maggie who he rescued from a Raider attack that killed both her parents.
If you read the terminal in Moriarty's Saloon you can read Colin Moriarty's thoughts on Billy. He believes Billy was a slaver who killed Maggie's parents and kept her as a trophy, however there is no other evidence in the game to support Colin's theory.