Kevin is a twelve-year-old seventh grader and the youngest of five boys in the Jamison family. Feeling ignored by his busy doctor parents and mistreated by his older brother Petey, Kevin channels his frustration into bullying his peers at school. He secretly writes free-verse poems in a personal notebook to cope with his feelings of alienation and powerlessness. He frequently acts as though he is made of unfeeling stone to protect himself from emotional pain.

Mrs. Little is the elderly librarian at Kevin's middle school. While Kevin initially views her as just another strict authority figure trying to punish him, she pays close attention to the students. She notices Kevin's distress and attempts to offer guidance through books, reading, and shared poetry.

Robin is a short seventh-grade student who becomes the primary target of Kevin's early-school-year bullying. He initially suffers quietly under Kevin's teasing and physical aggression. When he finds an opportunity to turn the tables, he aggressively targets Kevin in return and exposes Kevin's private thoughts to their classmates.

Petey is a high school sophomore and the second-youngest Jamison brother. He plays in a band and frequently torments Kevin by calling him names, locking him in closets, and ruining his belongings. He acts out aggressively toward Kevin, a behavior that started after Petey felt embarrassed about crying in front of his younger brother during a tornado.

Paul is one of Kevin's older brothers. He studies psychology and often attempts to apply his academic lessons to his family dynamics. Unlike the rest of the family, Paul actively pays attention to Kevin, offering him advice and attempting to intervene when Petey takes his bullying too far.

Kelly is a seventh-grade classmate of Kevin and Robin. Kevin initially refers to her as "Freckle-Face Kelly" and makes fun of her appearance. She endures bullying from both Kevin and Robin but eventually shares a quiet understanding with Kevin when the rest of the class turns against him.

Principal Hartwick handles discipline at the middle school. He frequently calls Kevin into his office to issue scoldings, detentions, and suspensions for misbehavior. He enforces a strict zero-tolerance policy that complicates Kevin's ability to defend himself from later attacks.

Mrs. Smithton is a seventh-grade teacher who frequently catches Kevin causing trouble. She dislikes the anonymous found poems pasted around the school and routinely sends Kevin to the principal's office for his disruptive and unkind behavior.

Giant John is a physically large seventh grader. Though initially targeted by Kevin for bullying, he later joins Robin's newly formed group to retaliate against Kevin on the playground.

Kevin's mother works as a busy doctor who is currently up for a promotion. She is frequently absent from home and regularly ignores calls from the school. When she does interact with Kevin, she focuses strictly on how his behavior impacts her professional obligations.

Kevin's father is a doctor whose demanding on-call schedule keeps him away from the family. Kevin actively wishes his father were more like the involved television dads he watches on the weekends.

Patrick is Kevin's oldest brother. As a college student, he is mostly absent from Kevin's daily life, contributing to the large age gap that leaves Kevin feeling isolated at home.

Philip is one of Kevin's older teenage brothers. Like Patrick, he pays little attention to his youngest sibling, leaving Kevin to fend for himself against Petey's bullying.