Billy Lenz, is the main antagonist of the 1974 horror movie Black Christmas, as well as its remake (in which he earned his second name) - he was a deranged serial-killer who managed to escape a mental asylum while his half-sister/daughter Agnes (when Billy's mom raped him) started killing the girls in sorority house which was his old childhood home. He is portrayed by Nick Mancuso in the original and by actor Robert Mann in the remake.
There is almost no information given about the killer in the original movie. All that is known is that a person climbs into a sorority house, and starts making obscene and sexual phone calls to the girls downstairs.
From the phone calls, the audience discovers that the man's name is Billy, and he obviously is mentally unstable. Billy routinely repeats phrases over the phone that gives hints to his identity. It is assumed that he sexually abused his younger sister Agnes. And his parents abused him regularly.
Billy goes on a killing spree killing the girls in a delusional rampage. He attempts to kill a girl named Jess but she manages to lock herself in the cellar. Her boyfriend Peter gains access to the cellar and believing he is the killer making the phone calls, ends up bludgeoning him to death in "self defense" and she is later discovered by the police, making them believe Peter must have been the killer all along.
The police leave Jess to sleep and leave, while one cop waits outside for forensics to arrive. However the camera pans into the attic where the two bodies of Billy's victims are still there, and he can be heard saying "Agnes, it's me Billy"; the telephone then rings. The ultimate fate of Jess is left ambiguous to the audience.
Much more information is giving about Billy's back story in the remake. It's revealed that he has a rare liver disease that gives him jaundice. His mother and father hate each other, but his father cares about his son while his mother hates Billy for reminding her of her husband.
One Christmas, Billy's mother and her boyfriend murder Billy's father. They then bury the body under the house and lock Billy in the attic.
Billy never leaves the attic and makes it his home. When Billy is twelve, his mother rapes him, and becomes pregnant with his sister/daughter. She names her Agnes.
- After seeing the end of the film, studio executives asked director Bob Clark to change the ending. The proposed idea was to have the cops leaving Jess along with Chris, Claire's boyfriend. She wakes up and he says "Agnes, don't tell them what we did." Then he kills her. However, Clark refused and insisted that the ending be ambiguous.