We first meet Billy Knight in the beginning of Lethal White when the clearly troubled young man appears at the office in Denmark Street, begging for Strike’s help. Billy has dark hair, a thin face and “sunken eyes” and has an “extreme unkemptness” about him.

Although Strike surmises that Billy is “definitely mentally ill” and “possibly psychotic,” he listens to Billy’s claims that he witnessed a child’s murder when he was just a boy himself. Before Strike can get more information, Billy becomes spooked and flees the office. 

While searching for Billy, Strike meets Billy’s older brother, Jimmy, who unkindly says Billy is “fucked in the head” and explains he has Schizoid affective disorder. We learn that Billy and Jimmy grew up at Steda Cottage on the grounds of the Chiswell house where Billy was subject to abuse from their alcoholic father.  

Strike becomes determined to not only find Billy and ensure his safety, but to investigate Billy’s claims of murder. Robin recognizes Strike’s motives and tells him she understands that he “can’t rest easy until [he’s] looked into it.”

In the Lethal White TV series, Billy is portrayed by actor Joseph Quinn.