Beth is a sixth-grade student in Miss Kemp’s class at Woodrow Wilson Elementary School. She is an observant narrator who values social order and struggles immensely with a school assignment requiring her to find honest compliments for her disruptive classmate, Imogene Herdman. While generally cheerful and a loyal friend to girls like Louella, her sharp wit and initial prejudices reflect her community's critical attitude toward nonconformity.

Imogene is a student in Miss Kemp's sixth-grade class and a member of the notoriously unruly Herdman family. Often unkempt and largely unsupervised at home, she participates in her siblings' chaotic schemes and tells elaborate lies to frighten other students away from the teacher's lounge. Beneath her tough, disruptive exterior, she possesses a fierce resourcefulness and a surprising willingness to protect vulnerable children from bullies.

Alice is an overachieving sixth-grade student in Beth's class known for her perfect attendance, good grades, and immaculate grooming. She is highly self-absorbed and constantly seeks adult approval and praise. Entitled and lacking in empathy, she represents the town's rigid demand for order and frequently directs her snide judgments at Imogene Herdman.

Mrs. Bradley is Beth and Charlie's mother. She is highly involved in her children's lives and an active member of the PTA at Woodrow Wilson Elementary School. Possessing a biting sense of humor, she frequently criticizes the misbehaving Herdman family as well as the overly demanding Mrs. Wendleken, though she remains an engaged and supportive parent to her own children.

Mrs. Wendleken is an overly involved parent who constantly volunteers her daughter, Alice, for local productions and public service campaigns. She is intensely proud of her daughter's achievements and oblivious to her snobbery. Quick to express disgust, she loudly advocates for strict discipline and the total exclusion of the unruly Herdman family from community life.

Miss Kemp is Beth and Imogene's sixth-grade teacher at Woodrow Wilson Elementary School. She introduces the central class project requiring students to closely observe and compliment their peers. She attempts to maintain order in her classroom despite the disruptions caused by the Herdmans and the arrival of Louella's toddler brother.

Charlie is Beth's younger brother, who is in the same grade as Leroy Herdman. He frequently reports on the Herdmans' strange claims and activities at school, acting as an additional source of playground intelligence and gossip for his family.

Mr. Bradley is Beth and Charlie's father. He shares his family's sarcastic sense of humor, often making dry remarks about the lack of talent in the school show or joking about the Herdmans' wild behavior confusing the local newspaper reporters.

Louella is Beth's friend and a sixth-grade classmate. She frequently bears the burden of caring for her toddler brother, Howard, and is forced to bring him to school on a leash when her mother's childcare arrangements fail. She remains highly suspicious of Imogene Herdman's sudden interest in protecting her brother.

Howard is Louella's toddler brother who is not yet toilet trained or able to read. He is intensely attached to his dirty security blanket and prone to severe breath-holding tantrums when it is taken away, making him a target for bullies and a surprising recipient of Imogene Herdman's protection.

Gladys is one of the younger Herdman siblings. She is known for her aggressive behavior and creative extortion schemes, such as fashioning fake braces out of paper clips to scam other students and coercing a terrified second-grader into violently smashing walnuts on his head for the school talent show.

Leroy is a Herdman sibling in Charlie Bradley's grade. He brings a dead snake to school after being bitten by it, causing widespread panic and chalk-smeared chaos when he ties the reptile to a supply closet light string to frighten his teachers and peers.

Claude is one of the Herdman siblings. He is known for participating in various disruptive pranks, including causing a disturbance on the school bus by releasing baby turtles. He was previously kept on a leash by his family in lieu of a dog.

Ollie is a Herdman sibling who frequently engages in mischief with his brothers and sisters. He notably locks his brother Claude in a gas station bathroom and steals the key, forcing the local fire department to break down the door.

Ralph is the oldest of the Herdman children. Along with his siblings, he participates in the family's unsupervised, chaotic behavior, stealing small items and contributing to the town's general exasperation with his family.

Mrs. Herdman is the single mother of the six Herdman children. Abandoned by her husband, she works double shifts at a local shoe factory to support her family, leaving her with virtually no time to supervise her children or manage her chaotic, impoverished household.

Miss Philips is the Herdmans' court-appointed social worker and a friend of Mrs. Bradley. Rather than actively intervening to help the neglected children, she admits to merely driving by their messy house once a month, feeling overwhelmed and helpless in the face of their squalid situation.

Eugene is a short, nervous second-grade student who is frequently bullied. Pressured by Gladys Herdman, he enters the school talent show with an act consisting solely of smashing walnuts on his forehead, earning him the nickname "Hammerhead" from his cheering peers.

Boomer is a sixth-grade classmate of Beth and Imogene. He demonstrates a casual cruelty when he attempts to steal toddler Howard's security blanket just to watch the child turn purple from holding his breath.

Lester is a student and the son of a school bus driver. He is tricked by Gladys Herdman into buying and wearing paper-clip braces, and she terrifies him with elaborate lies about the eye doctor, causing him to fail an examination in a panic.

Mrs. Yeagle is a school bus driver and Lester's mother. She is outraged to discover the fake paper-clip braces Gladys Herdman sold to her son and loudly declares the Herdmans entirely unwelcome on her assigned route, Bus 6.

Donald is a student who is easily frightened by the Herdmans' rumors. He becomes so terrified of the imaginary, flesh-eating bugs that the Herdmans claim infest the school bus that he refuses to leave his house in the morning.

Maxine is Donald's older sister. She attempts to defend her brother and assert order at their bus stop when the Herdmans begin lurking there to silently protest being banned from riding Bus 6.

Joanne is a sixth-grade classmate of Beth. She draws Beth's name for Miss Kemp's class assignment and delivers sincere, positive compliments about Beth being cheerful, graceful, and fair, which provides Beth with a momentary boost of confidence.

Boyd is a student whose head gets stuck in a school bike rack due to the playground antics of Claude and Gladys Herdman. Imogene successfully helps him get unstuck, an act of resourcefulness that Beth witnesses.