Created by Timothy Bond and A. Roy Moore, the character was partly inspired by the urban legend "The Babysitter and the Man Upstairs", in addition to the crimes of George Webster and the serial killer William Heirens.

Born with severe jaundice due to liver disease, he is physically and emotionally abused as a child by his mother, Constance (Karin Konoval).

Years later, an adult Billy (Robert Mann) escapes and goes on a rampage with Agnes (Dean Friss) at their old family home, which has been converted into a sorority house.

[5][6][7] Hays based the novelization upon Moore's original draft, which expands upon Billy's actions, depicting the character's inner dialogue, and alterations to key sequences.

A major source of inspiration was the urban legend of "The Babysitter and the Man Upstairs",[i] which itself is based on the unsolved murder of Janett Christman, who had been babysitting for the Womack family in Columbia, Missouri.

the basic storyline describes a young woman who, while babysitting three children, is tormented by a madman who leaves threatening phone calls, later revealed to be coming from upstairs in the house.

[29][79] Mann attended two short auditions before he was cast in the role, with the filmmakers initially questioning him on his height, size, and his ability to endure small spaces and prosthetic makeup.

Classified by behavioral scientist and psychiatrist Sharon Packer and art historian Jody Pennington as a "faceless killer",[83] one of Billy's defining characteristic is his complete ambiguity.

[xi] In his essay on the 1974 film, Morten Feldtfos Thomsen writes on the use of subjective camera in creating a level of uncertainty and tension while "obscure[ing] the killer’s identity by excluding him from the image and confining him to a vaguely defined space outside of the frame.

As an unseen presence in point-of-view sequences and the phone calls, according to Thomsen, portrays Billy as "a quasi-embodied or even disembodied entity, simultaneously on-screen and off-screen".

[92] Film historian Martin Rubin noted similar characteristics with Bruce the Shark from Jaws, as both represent a remorseless, near omnipresent and omniscient force.

[94] Writing for the entertainment magazine and website Birth.Movies.Death, Brian Collins states that, as viewers, we are encouraged to try and solve the mystery of Billy: "we're not meant to figure anything out, but [...] we can't help but try".

Billy's obsessive rambling about Agnes or "the baby", Taylor comments, hints at a real or imaginary event where the character failed to protect a loved one.

[96] Implications of a traumatic past were also commented upon by SlashFilm writer Lee Adams, who writes that, in one of the phone calls, Billy takes on the personas of a mother and father questioning him on what happened to "the baby".

[19] Alternately, Taylor comments that the motivation behind Billy's killing spree could be seen within the victims themselves, who display negative associations with motherhood, such as promiscuity, negligence and lack of responsibility.

[96] This theory was echoed by Anton Bitel of Little White Lies, who described Billy as "a dark version of Santa Claus, meting out punishments to the naughty and the nice alike".

[35] Writer and director Morgan wanted a more defined killer for the 2006 remake,[59] abandoning the original character's ambiguity in favor of a more traditional slasher villain.

[63] As Jason Zinoman stated, Billy's lack of backstory was altered by Morgan for the remake, with the film going "back in time" to reveal the character's identity and motivations.

According to John Saxon, who portrayed Lt. Fuller in the original film, felt that Billy had a "naturalistic basis" rather than a supernatural one, representing the darkest part of humanity "tormented and was capable of committing horrific [acts]".

[102] Analyzing the original film, Bud Wilkins of Slant Magazine notes Billy embodied a more realistic and human killer in contrast to what he called "the unstoppable boogeyman that Michael Myers represents".

Described as "proto-incel" by Bloody Disgusting, Muñoz's interpretation of Billy explains that he views others as nothing more than toys for his amusement and manifests a deep-seated hatred towards the sorority girls after spending weeks secretly living in the house and observing them.

[12] While the remake would retain Billy's mental instability,[98][99] Morgan chose to add more details to the character, portraying him as being born with severe jaundice,[103][104] which turned his skin yellow.

[81] Morgan stated that the character's motivations arise from their twisted definitions of love and family, which Billy equated with violence after witnessing his father's murder, and the years of maternal abuse he suffered.

In their book Misogyny, Misandry, and Misanthropy, R. Howard Bloch and Frances Ferguson describe Billy as a representation of Freudian attributes, with Barb's murder with a glass unicorn a symbol of male empowerment (phallus).

[116] According to Bakhtin, the "carnivalesque" is characterized by its breakdown of hierarchy, social barriers, and prohibitions,[117] while the "grotesque body" he defines as one of degradation, unashamed excess, anathema to authority and pious austerity.

[118] Downey and Hastings' analysis highlights Billy as "an agent of carnival process", exemplified through not only his obscene language and his mocking of Jess' unborn child, but also his behavior functioning to undermine and challenge society norms.

[119] Downey and Hastings argue that Billy is a manifestation of Bakhtin's grotesque body, commenting on Clark's portrayal of the character as a shadowy presence that is "not whole" as a person, suspends the normal hierarchal distinctions and societal barriers through his abnormal behavior.

At the film's end, Barb and Phyl are mutilated in a fashion that depowers their sexuality, symbolically releasing them of their "socially imposed sex roles", which exemplifies Bakhtin's theory that carnivalesque "exalts the blurring and shifting of gender distinctions".

Complex's Matt Barrone pointed out that Scream franchise's Ghostface killer, who uses the same method of phoning his victims, would not have existed if not for Black Christmas and Billy.

In the 2006 mockumentary slasher film Behind the Mask: The Rise of Leslie Vernon, the title character was mentored by a "retired" killer named Eugene, played by Scott Wilson.