Grace is a 23-year-old Irish immigrant serving a life sentence at Kingston Penitentiary. At barely sixteen, she was convicted as an accessory to a sensational double murder involving her employer and the housekeeper. Beautiful, intelligent, and fiercely observant, she presents a composed exterior to the outside world, though she privately struggles with traumatic memories and fainting spells. She currently works as a housemaid and seamstress in the prison Governor's parlor, where she sews quilt blocks while recounting her life story.

Simon is a young, ambitious American medical doctor specializing in the burgeoning field of psychiatric treatment. Following the collapse of his father's textile business, he relies on his profession to support himself and his mother. He travels to Canada to evaluate Grace Marks, hoping a successful report will attract wealthy patrons to fund his planned mental health clinic. He maintains a professional exterior but privately resents the rigid social expectations of his class and often struggles to maintain his objectivity.

Mary is a practical, cheerful, and plain-spoken sixteen-year-old orphan who befriends young Grace. Acting as an older sister figure, she teaches Grace the realities of domestic service and society. She holds strong democratic principles and views herself as the equal of the wealthy classes she serves, believing that hard work determines a person's true worth.

Nancy is the pretty, dark-haired housekeeper and mistress of Thomas Kinnear. She hires Grace, who is initially drawn to her due to her physical resemblance to Mary Whitney. Vain and self-centered, Nancy enjoys a comfortable, elevated lifestyle that includes silk dresses, gold earrings, and piano lessons. Her open relationship with her employer makes her an outcast in polite society, and she is highly insecure about maintaining her position.

James is a rough, foul-tempered former soldier from Ireland who works for Thomas Kinnear. He deeply resents having to take orders from Nancy Montgomery, barely containing his contempt for her authority. He drinks heavily and harbors intense anger toward his employers, frequently voicing violent threats when he feels slighted or disrespected.

Thomas is a wealthy Scottish landowner living near Richmond Hill. He holds somewhat egalitarian principles, paying his servants well and treating them with an easygoing familiarity that Grace finds confusing. Despite his open affair with his housekeeper, his wealth, class, and gender largely shield him from the social ostracization that affects the women in his household.

Reverend Verringer is an intelligent, politically astute Methodist minister in his mid-thirties. He heads the committee of reformers and spiritualists seeking a pardon for Grace Marks. Pragmatic and cautious, he wants to ensure Grace is truly innocent before formally submitting a petition on her behalf, actively sponsoring Simon's psychiatric evaluation to gather evidence.

Jeremiah is a charming, witty traveling peddler who sells goods to domestic servants. He operates outside traditional social boundaries, crossing borders freely and making a living through charm and unconventional trades. He takes a liking to Grace, recognizing her vulnerability and offering her a rare chance at an alternative, nomadic life away from domestic service.

Jamie is an agreeable fourteen-year-old neighbor boy who frequents the Kinnear property. He plays the flute and develops an innocent, eager admiration for Grace during her time working there, frequently attempting to cheer her up when she is distressed.

Mrs. Humphrey is Simon Jordan's lonely landlady in Kingston. Abandoned by her husband and left without funds, she relies heavily on Simon's financial and emotional support. She struggles to maintain the household, frequently retreating to a darkened room when overwhelmed by her dire financial circumstances.

Lydia is one of the prison Governor's daughters. She is an upper-class young woman who actively seeks Simon Jordan's attention, using social gatherings and manufactured interruptions to practice her charms on him.

Dr. Jerome DuPont is a self-described neuro-hypnotist who frequents social and spiritualist gatherings in Kingston. He asserts that hypnotic sessions can alleviate amnesia, putting him at professional odds with Simon Jordan's psychological methods.

Dora is the pragmatic maid at Mrs. Humphrey's boarding house. She eventually quits due to unpaid wages and does not hesitate to spread rumors about her former employer's chaotic household around town.

Grace's father is an English stonemason who struggles to find work in Ireland. Driven by poverty and frustration, he becomes a violent alcoholic who terrorizes his family, beats his wife, and eventually forces a young Grace into domestic service specifically to confiscate her wages.

Grace's mother is a beautiful Irish woman who marries beneath her station. She suffers greatly under her husband's abuse and the physical burden of bearing nine children, ultimately passing away during the horrific transatlantic crossing to Canada.

Dr. Bannerling is the superintendent of the insane asylum where Grace spent fifteen months. He firmly believes she is a cunning, deceitful liar who feigned madness to escape prison, and he strongly advises Simon against advocating for her release.

Kenneth MacKenzie is a prosperous lawyer who made his early career defending Grace Marks. He successfully argued to save her from the gallows by painting her as a simple-minded, impressionable youth, though he privately harbors his own theories about her actual involvement in the crimes.

Simon's mother is a widow who relies on her son's financial support following her husband's death. She frequently writes to Simon, expressing concern for his health and urging him to abandon his psychiatric studies in favor of a lucrative marriage and a safe profession.

George is one of the young gentlemen of the wealthy Alderman Parkinson household. He falls ill during the holidays and stays home, receiving a great deal of attention from the household staff, including Mary Whitney.

Edward Murchie is a doctor and a personal friend of Simon Jordan. He serves as Simon's primary confidant, receiving detailed letters outlining Simon's psychiatric theories, methods, and personal frustrations regarding the Grace Marks case.