Amy Anne is a nine-year-old prolific reader living in Raleigh, North Carolina. She is generally quiet and keeps her true thoughts inside her head instead of speaking them aloud. Overwhelmed by the chaos of her household and her demanding younger sisters, she uses books and the school library as her primary refuge. Her passion for reading drives her to establish a secret lending library in her locker.

Mrs. Spencer is the influential president of the Shelbourne Elementary PTA and a prominent community volunteer. She bypasses standard school procedures to directly convince board members that certain children's books are harmful. While she believes she acts in the best interest of students' safety, her unilateral decisions threaten the intellectual freedom of the student body.

Trey is a fourth-grade student at Shelbourne Elementary known for his cartooning abilities. As the son of the PTA president leading the book ban, his presence causes anxiety for the students running the secret library. He is assigned as Amy Anne's partner for a social studies project on the First Amendment, which forces them to interact despite their past tension.

Rebecca is a fourth-grader and Amy Anne's closest friend at the beginning of the story. Because her parents are lawyers, she possesses a sharp awareness of rules, liability, and paper trails. She assumes the roles of chief financial officer and legal counsel for the secret library, providing logistical support and caution.

Mrs. Jones is the librarian at Shelbourne Elementary. She wears rhinestone glasses and polka-dotted dresses, taking her job as a provider of reading materials seriously. She advocates for intellectual freedom and proper administrative procedure, actively opposing the arbitrary removal of library materials while mentoring students.

Danny is a fellow fourth-grade student who joins the effort to read and distribute the challenged books. He provides the locker library with titles from his home and enthusiastically embraces the secretive nature of the operation. He takes on the role of alternative acquisitions for the student-run library.

Principal Banazewski is the head administrator at Shelbourne Elementary. She prioritizes maintaining order and appeasing influential parents, leading her to enforce the school board's book removals. She actively works to uncover and shut down the students' secret book circulation.

Jeffrey is a fourth-grade student struggling with grief following the death of his grandmother. His emotional pain manifests as anger and misbehavior at school, leading to a physical fight in the cafeteria. He becomes a patron of the locker library when Amy Anne recommends a book to help him process his feelings.

Alexis is the middle sister in the Ollinger family. She is highly focused on her daily ballet practice and frequently takes over the shared bedroom, leaving Amy Anne with little personal space. The household often caters to her schedule and demands.

Angelina is the youngest member of the Ollinger family, a five-year-old who insists she is a pony named Rainbow Sparkle. She demands constant accommodation for her pretend play and requires the most attention from her parents, further crowding Amy Anne out of the family dynamic.

Mr. Ollinger is Amy Anne's father, an opera-singing parent trying to manage a chaotic household. He tries to support Amy Anne's interests but often defaults to asking her to be the mature one when conflicts arise with her younger sisters.

Mrs. Ollinger is Amy Anne's mother, a busy professional who works from home in a cluttered office. She relies on Amy Anne to maintain peace in the house by conceding to the younger children's demands.

Janna is a popular student at Shelbourne Elementary who writes a note asking to borrow a banned book. Her interest proves to Amy Anne that the desire to read restricted titles extends beyond her immediate friend circle.

Known as MJ, Michael is a fifth-grader with strong computer skills. Danny enlists his help to create convincing fake book jackets to disguise the banned books as they circulate through the school.

Dav Pilkey is the real-life author of the Captain Underpants series, appearing as a guest speaker funded by the PTA. He advocates for reading freedom during his presentation and quietly supports the students' rebellion.

Mr. Vaughn is Amy Anne's language arts and social studies teacher. He facilitates classroom activities like Free Reading Friday and assigns the First Amendment project that forces Amy Anne and Trey to collaborate.

Nikolai is a third-grade student who wants to check out a banned book. His request prompts Amy Anne to create a formal waiting list for popular titles.

Coletrane is a second-grade student whose clumsiness in the cafeteria nearly exposes the secret locker library to the principal. His accident highlights the risks involved in their operation.

Flotsam is one of the Ollinger family's two large Rottweilers. He adds to the physical chaos and noise in Amy Anne's home environment.

Jetsam is the second Rottweiler in the Ollinger household. He contributes to the cramped and chaotic conditions that drive Amy Anne to seek refuge in the library.