Billy Boy (portrayed by Jon Sklaroff) was a minor antagonist in the second season of Gotham. Appearing during the Rise of the Villains arc in the episode "A Bitter Pill To Swallow", he was an assassin working for the criminal fixer known as The Lady.
When Tabitha Galavan put out a hit on Detective Jim Gordon, the Lady sent Billy Boy to carry out the hit. As Gordon arrived at Theo Galavan's penthouse, Billy Boy got into the elevator with him. He appeared to make small talk for a while, but then drew a piano wire and attempted to strangle the detective. Gordon fought back and was able to overpower Billy Boy, then interrogated him to find out who had sent him. Billy merely laughed, stating that if he failed then other assassins would come to finish the job.
Billy managed to sneak away while Gordon and Nathaniel Barnes were distracted and helped the other assassins access the penthouse. Though the assassins were slain, Billy was able to get behind Katherine Parks and threatened to kill her. However, Gordon's superb marksmanship allowed him to shoot Billy in the head before he could harm Parks.