Judy is an opinionated and passionate third-grader living in Virginia. She experiences intense highs and lows, often acting out her frustrations when faced with unfamiliar situations like a new teacher. She dreams of becoming a doctor and proudly collects Band-Aids to practice her medical skills on her doll. She is highly motivated by her specific interests, though her stubborn streak sometimes causes friction at home.

Stink is Judy's second-grade younger brother. He is a lovable, sometimes clumsy boy who desperately wants to be included in his older sister's activities and secret clubs. While his naive interference occasionally causes accidents that frustrate Judy, he remains enthusiastic and well-meaning. His innocent antics frequently force his family members to practice patience.

Rocky is Judy's best friend and a fellow third-grader in Mr. Todd's class. He is a relaxed, open-minded boy who enjoys buying candy and practicing magic tricks. He reliably supports Judy through her changing moods, happily joining her in mischievous schemes like catching toads. He also displays a welcoming attitude toward younger children and new classmates.

Frank is a sweet, generous third-grade boy who sits next to Judy at school. He faces early rejection from his desk-mate, partially due to a schoolyard rumor that he eats paste. He is a highly organized collector who carefully stores items like old comic books and miniature hotel soaps. His persistence and shared hobbies slowly bridge the gap with his skeptical peers.

Mr. Todd is the new third-grade teacher at the local elementary school. He is a patient and observant educator who quickly recognizes his students' changing moods. He uses positive reinforcement to encourage better attitudes, assigning a "Me Collage" project to help the class get to know one another better.

Judy and Stink's mother manages the busy Moody household. She encourages her children to spend time together, often gently insisting that her daughter include her younger brother in social outings. She deals practically with the morning complaints and arguments of her children.

Judy and Stink's father is a supportive parent who takes an active interest in his children's hobbies. He encourages emotional growth and tries to help his kids fix their mistakes, even when those mistakes involve spilled juice on school projects. He accompanies his children on various outings, from pet store visits to school field trips.

Frank's mother is a welcoming woman who hosts her son's birthday party. She pays attention to the guests' interests and gives out thoughtful party favors. Her kindness helps spark new connections among the children in attendance.

Jaws is a Venus flytrap plant that serves as a unique new pet for the Moody household. The plant requires a diet of bugs and raw hamburger, providing entertainment for the children who feed it. Its changing energy levels visually mimic the emotional ups and downs of its owner.