Morgan created an extensive subplot for the film's killer, Billy, which introduced a secondary killer in Billy's younger sister, Agnes (in the original film, the names Billy and Agnes figure prominently in the obscene calls received by the sorority sisters). In writing the character of Billy, Morgan was inspired by the life of Edmund Kemper, a real-life serial killer who as a child had been locked in the basement of his home by his mother, whom he later murdered. The introduction of Agnes as a second killer was, according to Mary Elizabeth Winstead, incorporated in-part to extend the film's narrative arc, given that the "phone calls coming from inside the house" plot twist from the 1974 film was too familiar for contemporary audiences. The relationship between Billy and Agnes also allowed Morgan room to explore themes of family, which are also prevalent in the relationships between the female characters. In a 2014 interview with Morgan, he claimed that his inclusion of a second killer was under the urging of Dimension Films executives: "I felt that a ten million dollar movie of "Black Christmas" didn't need anyone's help, and they should have left us alone. But they had to have the two killers, and then they were after kids from "The O.C." We compromised a lot."