Peter is a ten-year-old fifth-grader who loves his second-floor bedroom, where he has lived his entire life. He enjoys reading, playing board games like Risk, and hanging out with his two best friends. When his parents force him to give up his cherished room for his grandfather and move to the spooky third floor, he feels stripped of his autonomy and becomes fiercely determined to reclaim his territory.

Grandpa Jack is a retired home builder who recently moved from Florida to live with his daughter's family. Dealing with the grief of losing his wife and suffering from a poorly healed leg injury, he spends his early days in the house staring into space. The family moves him into Peter's second-floor bedroom to spare him from climbing extra stairs, placing him at the center of a territorial dispute.

Jennifer is Peter's younger sister who loves ballet, candy, and sharing secrets. She wears clothes adorned with bold logos and acts as a typical younger sibling with a highly assertive personality. She remains oblivious to Peter's deep attachment to his bedroom and is simply thrilled about their grandfather moving into the house.

Arthur is an accountant who works long hours, particularly during tax season, often retreating to his dingy basement office. He takes a pragmatic approach to family decisions, prioritizing practicality over emotional attachments. He imposes the bedroom swap on Peter, expecting his son to accept that life is not always fair.

Sally is a homemaker who brings her father up from Florida out of deep concern for his physical and emotional well-being following her mother's death. She acts out of compassion for her father but fails to anticipate the intense emotional toll the room change will take on Peter. She insists Jennifer's bedroom remain close to the master bedroom, sealing Peter's fate to move to the third floor.

Billy is one of Peter's best friends since kindergarten. He is physically strong, capable of doing fifteen chin-ups, and frequently dominates games of Stratomatic Baseball. When he learns about Peter losing his room, he advocates for aggressive, direct retaliation against Peter's grandfather.

Steve is Peter's studious best friend, a voracious reader who loves school and memorizes vocabulary words on index cards. He wears glasses and avoids physical activities, preferring to rely on his mind. He treats Peter's family conflict as a political exercise, suggesting guerrilla warfare and referencing historical events like the American Revolution.

Mrs. Klein is Peter's fifth-grade English teacher. She assigns her class the task of writing a "true and real" story. This assignment prompts Peter to document his experiences with his grandfather and the conflict over his bedroom.