Peter is an eleven-year-old boy living in Manhattan whose routine is disrupted by a new sibling and a temporary move to suburban New Jersey. He prefers stability and familiarity, making the sudden changes in his family dynamic a source of deep frustration. Over time, he begins accepting his growing responsibilities, taking charge of his dog, Turtle, and helping care for his younger sister.

Fudge is Peter's highly intelligent and mischievous four-year-old brother. He demands constant attention, asking bold questions and speaking his mind without any filter. He possesses a strong sense of identity, fiercely defending his chosen nickname against anyone who attempts to use his legal name.

Jimmy is Peter's fiercely loyal best friend from Manhattan. He carries lingering anger stemming from his parents' divorce, making him emotionally sensitive and occasionally combative. He acts as a vital anchor for Peter, maintaining their bond through visits and a special Kreskin's crystal he gives Peter to help him sleep.

Mom is a hardworking woman who balances part-time work with the exhausting demands of raising her children. She shoulders the primary emotional and physical labor in the household, sometimes pushing herself to tears from pure exhaustion. She relies on a sense of humor and deep patience to manage her sons' chaotic behavior.

Dad is Peter's father, an advertising agency employee who quits his job to move the family to Princeton and write a book. He is emotionally distant, preferring to lecture his children rather than engage in meaningful conversation. His career pivot acts as the primary catalyst for the family's relocation stress.

Tootsie is the baby sister whose impending birth initially drives Peter to consider running away. As she learns to crawl, walk, and vocalize over the year, she forces the entire family to adjust their routines. She proves to be a source of joy, helping Peter discover the positive aspects of being an older brother.

Alex is an agreeable boy who becomes Peter's first friend in New Jersey. He helps Peter adjust to suburban life by showing him around town and inviting him into a small business digging up and selling worms to a neighbor. His relaxed demeanor contrasts sharply with Peter's typical emotional intensity.

Daniel is a precocious, outspoken six-year-old boy who becomes Fudge's first local friend. He possesses a tough attitude, frequently challenging others to arguments over minor details. He leads Fudge into serious trouble, demonstrating a distinct lack of remorse when caught breaking the rules.

Grandma provides practical support and warmth during the chaotic transition of Tootsie's arrival. She gives individual attention to each of her grandchildren, sharing secret dances with Peter and listening patiently to Fudge's complaints about middle-child syndrome.

Mr. Green is the principal at the boys' school in Princeton. He manages Fudge's rebellious behavior with diplomacy, transferring him to a new classroom to avoid conflict and smoothly handling a public prank during a school assembly.

Joanne is a girl in Peter's sixth-grade class in Princeton. She acts as Peter's first real romantic interest, holding his hand during a movie and sharing a quick kiss with him under the mistletoe during the holidays.

Uncle Feather is a cultured myna bird that Fudge chooses as his personal pet. The bird learns to speak French and trade insults, providing Fudge with dedicated attention while becoming a frequent source of annoyance for Peter.

Turtle is Peter's devoted pet dog, named in memory of a small turtle Fudge once swallowed. He acts as Peter's first real responsibility, relying on him for walks and care while fiercely protecting Peter from neighborhood bullies.

Mrs. Muldour is an older neighborhood woman in Princeton who mysteriously purchases live worms from the local boys. She possesses a playful sense of humor, playing along with Fudge's blunt questions by offering him a worm cookie on Halloween.

Mr. Bogner is Peter's new teacher. Peter immediately appreciates him, noting that he seems intelligent and capable compared to his previous experiences with educators.

Brian Tumkin is a famous children's author and Fudge's personal favorite writer. During a school assembly, he unwittingly participates in Fudge's prank by drawing an unflattering portrait of the principal based on Fudge's directions.