Diamond is a young, gentle boy living with his working-class family in rooms above a coach house in London. Named after his father's favorite horse, he has a pale face, fair hair, and a somewhat sickly constitution. He possesses an infectious optimism and a kind worldview that profoundly impacts the people around him. He readily assists his parents with household chores and reaches out to help strangers across the city.

The North Wind is a mysterious supernatural entity capable of changing her shape and size at will. She most often appears to Diamond as a tall woman with long dark hair, though she can shrink into a tiny creature or transform into animals like a wolf. She carries out difficult atmospheric tasks across the world while maintaining a gentle, protective attitude toward the young boy.

Mother is a busy housewife striving to manage a working-class household in Victorian London. She displays deep concern for Diamond's health, checking on him during the night and keeping him indoors during poor weather. She bears the emotional weight of the family's financial struggles but consistently demonstrates generosity toward others in need.

Also known as Joseph, Father is a hardworking coachman who values fairness and loyalty above all else. He takes immense pride in his work and cares deeply for the horses he drives. He expresses his love for his family by teaching Diamond practical skills, passing down his straightforward honesty and gentlemanly demeanor.

The Narrator is an unnamed tutor who works for a family living near the countryside. He eventually crosses paths with Diamond late in the boy's childhood. He possesses a gentle demeanor and takes the time to listen to Diamond without judgment.

Nanny is a young street sweeper who lives a difficult life under the care of her abusive grandmother. Her rough exterior and coarse language stem from her harsh environment, but she harbors a hidden sweetness. Her interactions with Diamond gradually introduce her to kindness, prompting her to soften.

Mr. Raymond is a wealthy gentleman and author of children's stories. He takes an active interest in Diamond's development, offering the boy a book and a monetary reward if he learns to read. He uses his resources to assist Diamond's family through times of sickness and hardship.

Mr. Coleman is an affluent man who employs Diamond's father as a personal coachman. He lives in a large house with a nice garden but resorts to dishonest business practices when faced with mounting financial troubles.

Miss Coleman is the sickly daughter of the wealthy Coleman family. She forms a gentle acquaintance with Diamond, allowing the young boy to play in her garden when she feels well enough to be outdoors.

Mr. Evans is a gentleman who acts kindly toward Diamond during a rough day in the city. He carries a complicated past connected to the Coleman family, directly influencing his current standing in London.

Old Diamond is the reliable, sturdy horse originally owned by the Colemans. He shares a deep, affectionate bond with Diamond's father and serves as the literal foundation for the family's cab-driving livelihood.

Ruby is Mr. Raymond's horse, who temporarily comes to live with Diamond's family. He enjoys an easy life and prefers resting over working, leading to humorous conflicts with the more stoic Old Diamond.

Old Sal is Nanny's cruel grandmother. She struggles with alcoholism and frequently mistreats Nanny, going so far as to lock the young girl out on the dangerous London streets at night.

Jim is a young shoeshine boy who maintains a close friendship with Nanny. He relies on street smarts to survive and feels fiercely protective of his independence in the city.

The unnamed cabman is a neighbor in the mews whose drinking causes misery for his household. Following a quiet intervention where young Diamond comforts his crying baby, he attempts to change his ways and becomes an unexpected ally at the cab stand.

Mrs. Raymond is a kind woman who marries Mr. Raymond. She helps facilitate the cab-driving family's move to the countryside and eventually employs Diamond to work inside her household as a page.

Dulcimer is Diamond's infant sister. The young boy invents the name for her before she is officially christened, and he spends much of his time nursing her and singing original songs to keep her quiet while their father rests.