Suds is a gentle boy entering the fourth grade. He cherishes his childhood comforts, specifically taking soothing baths, playing with his teddy bear Winky, and carrying a familiar first-grade lunchbox. He is highly empathetic and feels distressed when asked to harm or intimidate others. The sudden expectation to behave like a rebellious "rat" causes him immense stress.

Joey is Suds's best friend. He actively embraces the aggressive, rule-breaking persona expected of fourth graders. He equates rebellious behavior and external toughness with manhood, adopting a military buzzcut and temporary tattoos. He consistently pushes Suds to drop childish habits and act tough.

Judy is a superficial fourth-grade girl who captures Suds's affection. She pays attention strictly to boys who exhibit public feats of toughness or hold high social status. Her desire for popularity makes her a primary reason for the boys' changing behaviors as they try to impress her.

Suds's mother is a kind, humorous presence in the Morton household. She acts as her son's primary sounding board, listening to his fears about growing up. She consistently challenges the idea that aggression is necessary for manhood.

Suds's father is a comforting figure in the Morton household. He provides an alternative model of adulthood for his son, showing that grown men can express their emotions openly and without shame.

Gerald is a sixth-grade student who serves as the schoolyard bully. He uses his physical size to intimidate younger students, and his public mocking of Suds's lunchbox severely escalates Suds's social anxieties.

Amy is Suds's sister, frequently referred to by her nickname Zippernose. She takes pleasure in annoying her brother, often leaving mousetraps on his pillow as a running joke about his fourth-grade status.

Bubba is Suds's baby brother. He contributes to the lively and occasionally chaotic atmosphere of the Morton family household.

Mrs. Peterson is Joey's mother. She handles her son's sudden rebellion with a watchful approach, temporarily allowing him space to test his boundaries before enforcing household rules.