Billy Thompson, voiced by Matt Stone, is a South Park Elementary student, during the 1970s, whose only appearance is in Season 14's Insheeption. He is Mr. Mackey's bully and he is always seen with two un-named friends who presumably like to bully Mackey as well.
Billy has blonde hair with a yellow singlet and light green trousers with a black belt.
Billy's only appearance in the episode "Insheeption" where he is in Mr. Mackey's dream, who is trying to remember why he is traumatized by that day. He was also Mr. Mackey's bully throughout his time in Elementary school back in the early 1970s. He is noticeably more pissed at Mackey during the episode because Mackey snitched on him for smoking. Billy threatens to "do things you'll never forget!" to Mr. Mackey the next day during a field trip to the forest. The next day, he volunteers to join Mr. Mackey's group so he can bully him to which Mr. Mackey (and Stan and a sheep herder who are with Mr. Mackey in the dream) runs off into the woods in terror. As Mackey is running off, Stan stops him and tells him to face his fear so they can get out of the dream. Mackey finally agrees and prepares to fight Billy to which seconds later, the cast of Inception interrupts and kills Billy and his two friends, thinking they were the cause of Mackey's traumatization during that day (to which, it turns out they weren't). Because the sheep herder died in mr mackeys dream and was seen dead in the chair when everyone had awoken, it was likley that he died inreal life to.