Billy is an unknown serial killer and the main antagonist of the 1974 psychological horror film, Black Christmas. Though it's never stated, Billy might have a shared history with #6 Belmont Street, hinted by the toys in the attic and his maniacal phone calls. Prior to the events of the film, Billy had already been harassing the sorority sisters who lived in the house, cursing them and making violent threats against them. He is portrayed by both Bob Clark and Nick Mancuso.
Little is known about the killer, or even if his name is really Billy; his physical appearance is also shrouded in mystery, as only his hands and an eye are seen throughout the film, besides a shadowy silhouette when he murders Barb. It is hinted at the beginning of the film that Billy had already been calling the house for weeks, or possibly even months, before his initial killing spree, as the girls had nicknamed him the Moaner, due to his raspy wailing voice. It is possible that Barb interrupting him during the first phone call pushed him over the edge, though this is unlikely, as Billy had already murdered a local girl in a park earlier and possibly raped another a couple of weeks earlier, according to Clare.
Clues are given about Billy's backstory through the phone calls he makes to the sorority house, and it's even implied he once lived in the house with his parents and a younger sister named Agnes, due to the toys seen in the attic of the house and the fact that Billy had already been calling the house prior to the events of the film and even knew the address. It is hinted that Billy might feel regret for his actions, as he asks Jess to stop him, before flying into a deluded rage. Billy's parents had known he was mentally unstable and his father became angry when he discovered that his wife had left their son alone with their young daughter, Agnes, who was missing. It is unknown what Billy did to Agnes, but his phone calls imply some sort of sexual assault took place, or possibly a murder.
Billy refers to all of his female victims as Agnes. During the climax of the film, Jess discovers Barb and Phyllis's maimed bodies in an upstairs bedroom, before she is attacked by Billy, who chases her through the house. Jess is able to lock herself in the basement, before her boyfriend, Peter, arrives. Believing him to be the killer, Jess bludgeons him to death with a fireplace poker, before going into shock. The police conclude that Peter was the murderer and leave Jess alone in the house to sleep, unaware of Billy's presence or the bodies of Clare and Mrs. MacHenry in the attic. It is unknown if he ever succeeded in killing Jess, as the phone continues to ring at the end of the film.
- After seeing the ending of the film, studio executives asked director Bob Clark to change the ending. The proposed idea was to have the police leave Jess alone with Chris Hayden, Clare's boyfriend. She wakes up and he says, "Agnes, don't tell them what we did." before murdering her. However, Clark refused and insisted that the ending be left up to the audience's interpretation.
- It is implied through dialogue that Billy had committed other crimes before his initial killing spree, such as the murder of a local girl in a park and the rape of another young woman weeks earlier, according to Clare.
- It is unknown how long before the events of the film that Billy had been calling #6 Belmont Street.
- Bob Clark revealed a backstory for the character in a documentary for Black Christmas (1974). The character was the mentally unstable son of two parents, whom he murdered on Christmas eve. He then locked his baby sister, Agnes, up in the basement, but she escaped and went to the police, resulting in him being sent to a psychiatric hospital.