Donald is a highly enthusiastic and uncoordinated young boy with an oversized sense of wonder. He approaches life with zesty gusto, wearing a three-foot-tall giraffe hat to school and laughing uncontrollably at funny-sounding words. Born with an inverted stomach valve, he frequently throws up but handles the physical issue cheerfully. He views every day as an adventure and assumes everyone around him shares his good nature.

Mrs. Zinkoff is Donald's patient and understanding mother. She manages her son's eccentricities with creative parenting, such as keeping a supply of silver star stickers to reward him for good days. When Donald believes the stars fall from the night sky, she supports the illusion by pretending to cool them in the freezer before giving them to him. She balances nurturing his unique spirit with enforcing necessary boundaries.

Mr. Zinkoff works as a mailman, a profession his son deeply admires. He does not earn a high salary and tends to drive old, unreliable cars that he affectionately calls honeybugs, though his family privately refers to them as clunkers. He encourages Donald's growth by engaging his imagination with silly nicknames and allowing him to experience a pretend version of mail delivery on Sundays.

Polly is Donald's younger sister. As a toddler, she enjoys the constant activity in the Zinkoff household. She serves as an unintentional guard when Donald is recovering from surgery, reliably shouting whenever he attempts to sneak out the front door. She participates eagerly in family games regardless of her skill level.

Andrew is a boy Donald's age who moves into the house next door. He exhibits a spoiled and grumpy demeanor, struggling to adjust to his new environment. When Donald attempts to welcome him with a giant homemade cookie, Andrew reacts poorly to the sloppy presentation. He places high value on social status and competition.

Hector is a quiet, eccentric classmate who keeps entirely to himself and claims his pet lizard as his only companion. He constantly chews licorice and focuses intently on his unusual hobby of collecting enough of his own earwax to eventually forge a candle. His isolation makes him a prime candidate when Donald decides he needs to find a best friend.

Claudia is a young toddler living in Donald's neighborhood. Her mother keeps her on a leash during their walks to prevent her from wandering into the street. She develops a habit of handing Donald whatever objects she finds on the ground, including pebbles and discarded chewing gum, whenever he stops to speak with her.

Gary is a highly competitive student at Satterfield Elementary who excels in sports. He takes physical activities very seriously and expects his teammates to perform at a high level. He cares deeply about winning championships and establishing social dominance through athletic success, making him highly critical of his uncoordinated peers.

Miss Meeks is a veteran first-grade teacher who is exactly one year away from retirement. She views education as a grand adventure, referring to her required days of schooling as the Learning Train. She genuinely likes her students' enthusiasm but struggles to maintain classroom order when Donald goes into prolonged fits of giggling over her made-up word.

Mr. Yalowitz is a fourth-grade teacher who takes a distinctly encouraging approach to education. He recognizes Donald's cheerful spirit and moves him to the front row, actively looking for opportunities to praise him. He acts as a champion for underperforming students and attempts to build their confidence through prominent roles in class activities.

Mrs. Biswell is a strict second-grade teacher who fundamentally dislikes children and demands absolute neatness. She expects brilliance from her students and views Donald's terrible handwriting and incorrect answers as intentional mockery. She possesses a prized chalkboard eraser that she guards jealously against student use.

Mrs. Shankfelder is a fifth-grade teacher at Satterfield Elementary. She administers a psychological questionnaire to her students that asks them to identify their preferences and name their best friend. This assignment causes significant reflection for some members of her class.

Uncle Stanley is Donald's cheerful relative who drops by the house periodically. He brings a bright, optimistic energy to family gatherings. He enjoys spending time with his nephew and willingly participates in lengthy, casual games of Monopoly with the neighborhood children.

The Waiting Man is a gray-haired neighborhood resident who maintains a constant vigil at his front window. He spends his days looking out at the street, waiting for the return of his brother who went missing in action during the Vietnam War decades earlier.

The Old Lady is an elderly woman who lives on Willow Street and uses an aluminum frame to walk. She kindly plays along with Donald's pretend mail delivery, officially dubbing him her mailman. She offers a quiet, non-judgmental presence in the neighborhood.

Cherise is the mother of Andrew Orwell and the new next-door neighbor to the Zinkoff family. She handles the stress of moving into a new home while trying to manage her son's foul mood about the relocation.