Peter is an intelligent, analytical nine-year-old living in a New York City apartment on West 68th Street. As a responsible fourth grader, he frequently functions as a mature helper for his overwhelmed parents. He feels frustrated by the disproportionate attention his toddler brother receives but generally remains obedient and helpful. He highly values his personal space, his friendships, and his status as a pet owner.

Farley Drexel Hatcher, affectionately known as Fudge, is a two-year-old toddler with boundless energy and a talent for creating chaos. He constantly commands the attention of his parents through food strikes, wandering off, and destroying property. Despite his disruptive habits, he deeply admires his older brother and constantly attempts to imitate him. He harbors a particular, dangerous fascination with Peter's forbidden bedroom.

Ann Hatcher is a dedicated mother managing a busy Manhattan apartment. She often finds herself exhausted by the constant physical and emotional demands of rearing an unpredictable toddler. To maintain peace during stressful moments, she regularly co-opts her older son into pacifying the younger one. She occasionally reacts emotionally under stress but genuinely loves and values both her children.

Warren Hatcher works as an account executive creating television commercials. He is a busy professional who occasionally brings his work stress home, seeking to impress demanding corporate clients like the owners of Juicy-O and Toddle-Bike. While he lacks daily hands-on childcare expertise, he steps in to enforce strict discipline when his toddler crosses boundaries his wife cannot manage.

Jimmy is Peter's reliable best friend and fourth-grade classmate. He is an urban child who knows how to spot danger in Central Park, having already experienced muggings in the area. He shares Peter's interests and provides a loyal, easygoing contrast to the stress Peter frequently experiences at home.

Sheila is a bossy, assertive classmate who lives in Peter's apartment building. She frequently seeks Peter's attention through antagonistic teasing, loudly accusing him of having "cooties." Underneath her confident, controlling exterior, she easily becomes frightened and tearful when faced with actual responsibility.

Dribble is a small pet turtle belonging exclusively to Peter. He lives in a bowl on a dresser and represents the boy's first real opportunity to demonstrate responsible pet ownership. The turtle remains completely unaware of the constant threat posed by the curious toddler living in the apartment.

Henry is the friendly elevator operator in the Hatchers' apartment building. He possesses an encyclopedic memory for residents' names and floors. He treats the building's children with respect, kindly allowing them to use his first name.

Mr. Yarby is the president of the Juicy-O beverage company and a crucial client for Mr. Hatcher's advertising firm. He is a strict, traditional businessman who holds rigid expectations about childhood manners and proper family conduct.

Mrs. Yarby is the fastidious wife of the Juicy-O president. Childless and highly excitable, she expects absolute household order and quickly develops a profound distaste for Fudge's chaotic behavior.

Peter and Fudge's grandmother provides occasional childcare assistance for the busy family. She offers a comforting presence and uniquely taught Peter how to stand on his head during a previous visit.

Mr. Vincent is the cigar-smoking president of the Toddle-Bike company. He is an aggressive tycoon who uses his financial leverage to force the ad agency to meet his exact, impulsive demands.

Janet is Mr. Hatcher's young, heavily made-up secretary. She is tasked with entertaining Peter and Fudge during their office visit, utilizing snacks from her purse to pacify the children.

Ralph is an overweight three-year-old boy who attends Fudge's birthday celebration. He particularly enjoys eating frosting and cake to excess.

Jennie is a young toddler going through a phase where she frequently bites people. She is unafraid to express her physical needs or intentionally create messes.

Sam is a highly fearful three-year-old boy. He bursts into tears at sudden noises, finding balloons and musical toys terrifying.

Mr. Berman is a shoe salesman who assists the Hatcher family. He remains professional even when dealing with difficult, uncooperative toddlers.

Dr. Brown is the family dentist. He utilizes clever psychological tricks, like asking Peter to model good behavior, to convince uncooperative young patients to open their mouths.

Dr. Cone is the family pediatrician. He provides practical, calm medical advice regarding childhood injuries, diet strikes, and emergencies.

Mrs. Haver is Peter's supportive fourth-grade teacher. She creates an encouraging academic environment where diligent students receive praise for their hard work.