Jack Speight moves with his wife, Rita, into Grandma's third-floor apartment. Everyone—Ma, Dolores, even Grandma—falls for the "tall and blond" (5.1) disc jockey. Grandma even buys a radio to listen to his show. We almost fall for him, too. He's young, attractive, and charming, and he gives Dolores rides in his car, making this sad kid feel too cool for school.
But things get weird fast. He tickles her feet (which eventually gives her PTSD whenever someone else touches her feet), for instance, and he repeatedly asks if she is good "at keeping secrets" (6.75), mainly because he wants to get away with assaulting her and not get caught. He gets progressively moodier when Rita gets pregnant, and angrily lashes out at Dolores.
When Jack gets fired (men in this book do not handle getting fired well), he takes Dolores to an old junkyard where dogs are chained up and rapes her on the ground. Then he kicks her and "talk[s] softly to the dogs, soothing them" (7.155)—"These are the poor f***ers nobody wants" (7.95), he says. You know, like he's a nice guy who really cares about mutts instead of a jerk who just raped a young girl. In Jack's case, we can see why no one wants him.
Jack leaves after Dolores tells Roberta what happened. We never see him again, but what he does to Dolores affects her for the rest of her life.