Grandpa Jack is the antagonist only in the sense that he’s in conflict with Peter over occupancy of Peter’s bedroom. Grandpa clearly loves Peter, and they enjoy activities together. Still, Grandpa and Peter experience conflict—they’re at war. As Peter tells him, “You’re my enemy” (104).

After Peter’s announcement that Grandpa is his enemy, Grandpa slaps him. After he hits Peter, Grandpa realizes, “I had darn near lost my whole sense of humor” (128). He participates in the war and keeps it going by launching counterattacks. He is ambivalent about the war; on one hand, he is frustrated and wants Peter to understand the severity of war; on the other hand, the pranks shake him out of his depression as he schemes to answer Peter’s pranks with pranks of his own. Ultimately, Grandpa and Peter discuss the escalation of their war.

Grandpa is also Peter’s mentor. He tries to teach Peter about war—real war. He tells him, “War hurts. War wounds and kills and causes misery. Only a fool wants war” (105). When Peter declares that Grandpa is his “enemy,” Grandpa slaps Peter hard. Peter is shocked—and Grandpa later regrets the act—but Grandpa wants to teach Peter that real war isn’t trivial, a game, or without pain. He also teaches Peter about fishing and building, letting him help transform the basement into his new apartment.

Grandpa transforms over the course of the novel. Initially, he is depressed. His wife has died, his leg hurts him, and he can’t build houses. His daughter has moved him into her house without consulting him about the arrangements, so he lacks power and agency. As he tells Peter, “[T]his situation is kind of out of my control” (90). He doesn’t socialize with his family, preferring to stare into space. Yet the war with Peter pulls him out of his doldrums. By the end of the story, he has found his voice and his agency; he speaks with Peter’s parents about turning the basement into the apartment, and then he showcases his might by building the apartment with help from friends and Peter. Though Peter sometimes presents Grandpa as “old and helpless” (173), Grandpa is capable, and he comes up with a solution that gives him his own space and agency and allows Peter to have his old room.

Jennifer is Peter’s little sister and his antagonist, and the role turns Jenny into something of a stock character—the stereotypical annoying younger sibling who is a bit spoiled. She bothers him about his story and breaks his rocking chair, and she reveals to Peter that Grandpa is taking over his room. Jennifer is bad at keeping secrets, and she’s assertive and willful. She has high self-esteem and lots of opinions. She gives Peter suggestions about what to include in his story, and she also tells him “to keep the sentences short” (13). Though she’s irritating, Peter isn’t mean to her, and he doesn’t ignore her. He incorporates her advice and tries to keep his sentences on short.

Peter describes one of his sister’s outfits: “Jennifer was wearing a Pac-Man cap, her Superman T-shirt, a jeans belt that said JEANS on it, and sneakers that said LEFT and RIGHT on the toes. She looked like a walking billboard” (14). Jennifer is a typical younger sister, and Peter’s portrayal of her conveys her likeability and his typical older-sibling annoyance. Smith’s audience can relate to their normal brother-sister relationship.

Arthur and Sally Stokes are Peter and Jennifer’s mom and dad. They’re stock characters—typical parents who might not leave much of an impression on the reader. Yet in Peter’s hyperbolic tone, the mundane parents become more than antagonists: They’re symbols of tyranny. They chose to give Peter’s room to Grandpa, and they didn’t discuss the decision with Peter or Grandpa. The origins of the war start with them. Through their top-down communication, the parents create conflict.

Arthur speaks to Peter about his feelings, but the dialogue is one-sided and discourages honesty. Concerning Peter’s discomfort with his new room, Arthur tells him, “I know. But, Peter, you mustn’t ever let Grandpa Jack know that. Or else he’ll feel terrible. And believe me, he feels terrible already” (43). Arthur wants to encourage empathy in Peter, but Peter first needs to be heard and validated. Grandpa sums up Peter’s parents’ behavior when he tells him, “Your parents took your room away and shut you up” (174). The parents make Grandpa and Peter feel like they lack agency and power. However, once the war is over, they listen to Grandpa and agree to let him turn the basement into his new apartment.

Arthur works as an accountant, and Sally is a homemaker. Arthur isn’t good at building things, as his dilapidated basement office indicates, and Sally, as Grandpa announces, is quite stubborn. Sally is also compassionate and worries about Grandpa (her dad). Peter sees her crying as she tells Arthur, “There’s just no life in him. No life” (53). Arthur replies, “It’s only a few months since she died. He’s very depressed. Give it time, hon” (53). The scene gives the reader a brief look into their personalities: Arthur understands Sally’s concern for her father and identifies the weight of Grandpa’s grief. They both want to support Grandpa, even though their approach has been imperfect. By the end of the novel, they also learn that effective communication with Grandpa results in a solution that gives him a purposeful task that will benefit not only himself but also the entire family.