Beth is an observant fifth-grader living in an Ohio town where she attends Woodrow Wilson School. She acts as the voice of reason among her peers, watching how the community responds to the threat of the Herdman siblings. She notes the differing attitudes between the adults who want a highly controlled holiday and the children who crave traditional scares. While she generally avoids trouble, she recognizes that a completely sanitized school event lacks the fun that makes the holiday special.

Charlie is Beth's impulsive younger brother and a fourth-grader who deeply appreciates the chaotic spirit of Halloween. Unlike his sister, he frequently finds himself caught up in the Herdmans' antics, operating as something between an accomplice and a willing victim. He is highly creative, working hard to build a two-person lion costume with his best friend.

Imogene is one of the notorious Herdman siblings, a group of unruly children who run wild around town with little adult supervision. She is tough and perceptive, serving as the primary spokesperson for her siblings at school. Her unpredictable nature keeps her classmates and the town's adult population on edge.

Alice is an overachieving, rule-following fifth-grader whose prim demeanor makes her a frequent target for pranks. She closely aligns herself with adult authority and openly despises the Herdmans. She plans elaborate, highly structured costumes and receives fierce protection from her equally strict mother.

Mr. Crabtree is the principal of Woodrow Wilson School who notoriously hates Halloween. He usually discourages costumes and decorations, but the mayor tasks him with running a heavily chaperoned school party this year. He seizes the opportunity to assign historical essays about the holiday instead of allowing traditional games.

Mrs. Bradley is Beth and Charlie's mother. She represents the adult perspective in town, feeling immense relief that the local authorities have organized a safe, controlled event. She volunteers on the pumpkin committee at the school to ensure the environment remains heavily supervised.

Louella is Beth's friend and classmate. She is highly anxious about the Herdmans' potential presence at the school party. She is motivated by the monetary reward her mother offers her for babysitting, provided the environment remains safe.

Alice's mother is a controlling and highly vocal member of the community who heavily influences the parent-teacher association. She holds a deep grudge against the Herdmans for past slights against her daughter.

Boomer is a classmate of Beth who initially struggles to get excited about the school-sponsored party. His elaborate costume plans reflect the children's desire to maintain some sense of traditional creativity despite adult restrictions.

Cecil is Charlie's good friend and a fellow fourth-grader. He agrees to be the back half of a two-person lion costume, managing the logistical challenges of wearing a slipcover with limited visibility.

Howard is Louella's infant brother. His presence at the school event requires Louella to remain vigilant while she pushes him in a stroller.

Leroy is one of the notorious Herdman siblings. His past pranks are legendary throughout Woodrow Wilson School, contributing to the town's collective fear of Halloween night.

Ralph is another Herdman sibling. His curious but chaotic ideas frequently lead the other children in town, including Charlie, into bizarre and highly disruptive predicaments.