Jack is the middle son of Strike’s sister, Lucy. According to Lucy, Jack “hero worships” Strike and wants to be a soldier like his uncle. We first meet Jack in The Cuckoo’s Calling when Strike attends Jack’s birthday party. Jack finds Strike in a room alone where he opens the soldier figure Strike bought for him. Lucy later finds them playing with the soldier, with Jack pretending to shoot Strike and Strike pretending to have been shot in the stomach.
In Lethal White, Jack becomes ill with a bad case of appendicitis. With his parents gone on an anniversary trip to Rome, Strike stays with Jack in the hospital until his sister and brother-in-law are able to return. Accompanied by Robin, Strike is clearly upset by the state of his nephew and resolves to become a more involved uncle.