Billy Thompson, voiced by Matt Stone, bullied Mr. Mackey when they were children. He made his only appearance in the Season Fourteen episode, "Insheeption".
In "Insheeption", during Mr. Mackey's dream, Billy threatened to beat Mackey up for telling on him for smoking. During a field trip, he volunteered to be in Mackey's group in order to bully him. However, Stan Marsh, who was trapped in Mackey's dream, convinces him to stand up for himself and fight Billy. Shortly before the fight, Billy and his friends were killed by several men who infiltrated Mackey's dream. However, Billy's murder was in a dream, so the incident didn't kill him in real life. It is unknown whether or not he is still alive to date.
Billy has blond hair. He wears a yellow singlet and light green trousers with a black belt.
Billy seems to be violent, as he was quick to threaten to beat up Mr. Mackey, despite Mackey only looking out for his health.