Bonnie is the passionate, hardy, and independent daughter of Sir Willoughby and Lady Green. Small for her age with brilliant blue eyes, she possesses a fiery temper and a strong, obstinate spirit that is balanced by her fierce loyalty. Having grown up with immense privilege and freedom, she reacts violently when her family's comfortable life is threatened, but she quickly learns to develop practical skills to survive hardship.

Sylvia is a delicate, initially timid orphan who travels from London to live with her wealthy uncle at Willoughby Chase. Having grown up in poverty, she is polite, cautious, and eager to please, closely following her Aunt Jane's advice to avoid strangers. Despite her early fears of the country and the adults around her, her innate kindness and growing bond with Bonnie draw out her courage.

Simon is a lithe, resourceful boy who lives in a cave on the Willoughby estate. Permitted to live there by Sir Willoughby, he makes his living by raising geese and selling them in London. Deeply connected to the natural world, he is a fiercely loyal figure of trustworthiness who provides warmth, food, and strategic help to the girls during their times of greatest need.

Miss Slighcarp is a tall, thin, severe woman clad in gray who is brought in to act as guardian for Bonnie and Sylvia while Sir Willoughby and Lady Green travel. Authoritative and cruel, she clearly favors Sylvia's meekness over Bonnie's impetuous nature. She quickly begins making drastic, unauthorized changes to the household to benefit herself.

Mr. Grimshaw is a mysterious stranger who meets Sylvia on her journey to Willoughby Chase. After suffering a head injury at the station, he is brought to the estate to recover. Although he initially appears helpful and claims to have amnesia, his secretive behavior—such as burning documents—soon reveals a darker, more dangerous purpose at the estate.

Mrs. Brisket is the cruel and imposing headmistress of an austere school for orphans. She runs an oppressive institution where children are overworked, starved, and treated with callous disregard, all while keeping up appearances for inspectors.

Pattern is Bonnie's devoted and gentle maid at Willoughby Chase. She acts as a constant, unwavering force of care for both Bonnie and Sylvia, contrasting sharply with the severe adults attempting to take control of the household. She proves brave enough to secretly assist the girls even when her own position and safety are threatened.

Aunt Jane is Sylvia's frail, elderly aunt who raised her from infancy. Living in poverty in a small London apartment, her declining health forces her to send Sylvia to live with her wealthy uncle. She is deeply loving, highly cautious of strangers, and serves as an important moral compass for Sylvia.

Sir Willoughby is the benevolent, wealthy owner of Willoughby Chase and Bonnie's father. He is a loving and indulgent parent who welcomes his niece Sylvia with open arms. His decision to travel abroad for his wife's health and leave the estate in the care of Letitia Slighcarp inadvertently leaves the children completely unprotected.

Lady Green is Bonnie's mother and the lady of Willoughby Chase. She is a gentle woman whose fragile health necessitates a long trip abroad, effectively removing her protective presence and fine clothing from the estate.

Diana is Mrs. Brisket's daughter, who assists in running the harsh orphan school. She shares her mother's cruel disposition and enjoys exercising petty, unchecked authority over the impoverished students.

James is a footman at Willoughby Chase who manages to retain his position after Miss Slighcarp dismisses the rest of the honest staff. He is deeply sympathetic to the girls' plight and tries to help them endure the newly hostile environment of their own home.

Emma is an orphan at Mrs. Brisket's school who is assigned to work with Sylvia. She provides the new arrivals with practical advice on how to survive the awful conditions and avoid Mrs. Brisket's wrath, demonstrating the quiet solidarity among the students.

Lucy is a pupil at Mrs. Brisket's school whose unevenly cut short hair and dingy, numbered overalls immediately signal the institution's harsh, dehumanizing conditions to the newly arrived girls.

Alice is an orphan at Mrs. Brisket's school whose disloyal behavior reflects the suspicion and discord the administration actively attempts to sow among the students to keep them divided.

Dr. Field is a capable physician who lives in the apartment below Aunt Jane in London. He proves to be a trustworthy adult ally to the children, offering medical expertise, food, and crucial assistance in untangling the legal problems they face regarding the estate.

Mr. Gripe is Sir Willoughby's lawyer in London. He is a professional man who initially accepts Miss Slighcarp's forged references but proves willing to seek the truth and intervene when presented with hard evidence of her deception.

Mr. Wilderness is a kindly smith and a friend of Simon's who lives in Herondale. He exemplifies the positive nature of strangers by offering the traveling children shelter, food, and restorative care without hesitation.

A large flock of white geese raised by Simon. They function as both his livelihood and his steadfast companions, serving as an early warning system against threats and a surprising source of warmth during perilous, snowy journeys.

The wolves are a constant, roaming physical menace in the snowy, alternate-history English countryside. Their presence disrupts train travel and severely endangers those caught outside, serving as a wild, natural reflection of the human dangers closing in on the children.