Arthur is a slender, pale thirteen-year-old boy struggling with intense grief following the recent death of his father in a motorcycle accident. Ordered to complete 120 hours of community service instead of going to juvenile detention, he initially resists his assigned task of collecting discarded items from the trash. He views the world through a lens of loss and anger but possesses a quiet capacity for growth.

Mr. Hampton is an elderly man known around the Washington D.C. neighborhood as the "junk man" due to his habit of collecting trash. He becomes Arthur's community service supervisor after requesting the boy work for him rather than go to jail. A World War II veteran, he secretly uses discarded materials to construct a massive artistic representation of heaven in a rented garage.

Squeak is a small seventh-grader who wears thick glasses and frequently attracts the attention of school bullies. After Arthur intervenes to help him, he proves to be a loyal and surprisingly courageous friend. He wraps his lunch in foil, inadvertently helping Arthur with his trash collection assignments, and approaches conflicts with calm intelligence.

Arthur's late father died in a motorcycle accident three months before the story begins. He had a history of run-ins with the law and rode a motorcycle, leading many authority figures to judge him harshly. Despite his troubled record, he possessed a knack for fixing things and creating careful, bright Christmas tree displays with his son.

Officer Billie is a strict, no-nonsense probation officer assigned to oversee Arthur's community service. She expects absolute compliance and does not accept excuses or failure from the youths under her supervision. Despite her tough exterior, she evaluates her probationers based on their current actions rather than their past mistakes.

Arthur's mother is a grieving widow attempting to hold her family together in the wake of her husband's sudden death. Overwhelmed by loss, she throws away many of her late husband's belongings to cope, inadvertently sparking Arthur's anger. She worries constantly that Arthur will adopt the worst habits of his late father.

Judge Warner is a self-important legal official who oversees Arthur's initial hearing. He bases his legal opinions heavily on family history, assuming Arthur will become a criminal simply because Arthur's father had a troubled record. He reluctantly allows Arthur to complete community service at Mr. Hampton's specific request.

Mr. Barber, known to the students as "Vice," is the dry, strict vice principal at Arthur's middle school. He preemptively relocates Arthur's locker away from the other seventh graders, believing the boy's criminal record makes him a bad influence. He consistently assumes the worst about Arthur's behavior during school disputes.

Groovy Jim is the laidback owner of a tattoo shop located near the garage where Mr. Hampton works. He provides Arthur with guidance and supplies early in the probation assignment, serving as a friendly neighborhood presence who recognizes the deeper wisdom in Mr. Hampton's unconventional lifestyle.

Roger is a balding, slightly overweight carpenter who begins dating Arthur's mother. He builds a wooden birdhouse for her and possesses practical skills for fixing broken items. Arthur initially resents his presence, feeling that he is replacing his late father.

Barbara is Arthur's seven-year-old sister who loves playing with Barbie dolls. She provides a sense of normalcy in the household after their father's death and frequently asks Arthur direct, innocent questions about heaven and mortality.

Tony is the landlord who owns the brick garage where Mr. Hampton builds his artistic creations. He is primarily concerned with collecting rent and clearing out the space for future tenants, presenting a practical threat to the existence of the artwork stored inside.