Jill is a fifth-grade student who collects stamps and bites her nails when anxious. She is a picky eater who strongly prefers peanut butter sandwiches over most other foods. At school, she participates in the bullying of her classmate Linda, acting out of a desire to appease the popular girl, Wendy. Jill often views the world through a rigid lens, deciding what kinds of punishments she believes other people deserve.

Wendy is a popular, highly influential fifth-grader in Mrs. Minish's class. She holds several leadership positions, including class president and recess captain, giving her significant authority over her peers. Wendy uses this social power to organize relentless bullying campaigns against students she dislikes, demanding absolute loyalty from her classmates. She is highly manipulative and capable of lying smoothly to adults.

Linda is a slightly pudgy fifth-grade student who becomes the primary target of her classroom's bullying. After presenting a report on whales, her peers brand her with a cruel nickname and subject her to daily verbal and physical harassment. Despite the constant torment, Linda tries to defend herself and often simply wishes to be left alone. Away from her bullies, she is talkative, friendly, and interested in topics like reincarnation.

Tracy is Jill's neighbor and best friend, though she attends a different fifth-grade class. She is a Chinese-American girl whose family keeps chickens and various pets at their home. Tracy avoids the toxic classroom dynamics of Jill's class, but she still engages in neighborhood mischief. She is observant and serves as an outside perspective on Jill's behavior at school.

Caroline is a fifth-grade student who functions as Wendy's loyal sidekick. She physically towers over many of her classmates, making her an effective enforcer for Wendy's bullying. Caroline rarely thinks for herself, preferring to echo Wendy's insults and follow her direct orders without question.

Kenny is Jill's younger brother, a fourth-grader with a deep love for trivia, world records, and unusual facts. He frequently annoys Jill by interrupting conversations to share his knowledge. Kenny is highly adaptable, easily brushing off teasing from older kids by enthusiastically sharing more facts.

Mom is Jill and Kenny's mother. She is actively trying to quit smoking, a habit she struggles with whenever she faces stress. She tries to impart moral lessons to Jill, warning her daughter to put herself in other people's shoes and to learn how to laugh at herself.

Dad is Jill and Kenny's father. He uses structured incentives to encourage good behavior, offering Jill a monetary reward for her stamp collection if she can stop biting her nails. He holds firm on punishments, insisting that his children face the consequences of their actions when they misbehave in the neighborhood.

Mr. Machinist is an older, grumpy man who lives in Jill's neighborhood. The local children dislike him because he refuses to donate to charity drives and generally acts hostile toward them. He does not tolerate neighborhood pranks and demands strict restitution when his property is damaged.

Mrs. Minish is the fifth-grade teacher for Jill, Wendy, and Linda. While she attempts to maintain order in her classroom, she is largely oblivious to the severe bullying happening right under her nose. She focuses heavily on students thinking the "right" way and following instructions to the letter.

Mrs. Sandmeier is the Brenner family's energetic housekeeper and caretaker. She is in her late fifties, athletic enough to beat the kids at basketball, and encourages them to practice speaking French.

Mrs. Rothbelle is the school's music teacher. She is easily offended and quick to hand out group punishments when she feels students are being disrespectful during her lessons or assemblies.