Billy Boy was a hitman at the disposal of The Lady.
Attempt on Gordon's life
Billy Boy was hanging out at The Lady's illegal casino and looking at some papers as Tabitha Galavan ordered a hit on Detective Gordon. The Lady accepted the job and sent Billy Boy to do it. Intercepting Gordon on his way to Galavan's penthouse, Billy Boy tried and failed to strangle Gordon in the elevator, after a small, seemingly friendly talk.
After that Gordon interrogated Billy Boy on the spot, even torturing him by hanging him out a window, but with no useful results. The only thing Billy Boy ever said about his employer was that she followed a very strict protocol of sending the new people until the job was done. When this proved true, Billy Boy helped other hitmen to sneak into the penthouse, remaining silent, and, using a surrounding mess, demanded a way out from Gordon, taking hostage and threatening to kill Officer Parks. Gordon then shot him in the head.
Despite an eccentric appearance, which includes easy-to-spot black-and-red jacket and red round glasses, Billy Boy is a professional killer who uses a piano wire as a garrote, which doesn't blow his cover as a tuner for Gotham Piano & Woodwinds.
He seems to have a habit of chatting in a friendly way with a victim, before killing, as a way to get them to drop their guard. Especially notable is a maneuver of offering caramel candy to a victim, and retrieving a weapon while putting it back in his bag (if the person refuses; theoretically, it works anyway, because accepting the person will have both hands busy unwrapping it). At least partially, though, it is not an act, as even beaten Billy Boy jokes and laughs a lot. Billy Boy is an exceptionally strong and agile hand-to-hand combatant, not only able to handle Jim Gordon, but also easily taking a serious beating from him.
- A harsh fight between Jim Gordon and Billy Boy, which takes place in the elevator may have been inspired by 2011 cult film "Drive", in which the main protagonist, The Driver, spots that a nice-looking man in a business suit is a hired gun (as his gun is seen in the jacket) and beats him to death. Here, however, the scene gets inverted.