Isobel is a young Scottish seamstress with synesthesia, a neurological condition that causes her to see letters and emotions as vibrant colors. Taught by her mother to hide this trait out of fear of witchcraft accusations, she channels her sensory experiences into intricate embroidery. She immigrates to Salem, Massachusetts, hoping to establish a stable life and a dressmaking business, but quickly finds herself struggling to survive in an unfamiliar, prejudiced society.

Nat is a brooding, aspiring writer living in Salem after the death of his father. He is fascinated by local history and deeply troubled by his ancestor's role as a judge in the Salem Witch Trials. Prone to romanticizing the world around him, he views Isobel as a magical figure he has conjured from his own imagination, incorporating her essence into his dark stories.

Edward is an apothecary who is thirteen years older than Isobel. He marries her on the promise of a comfortable life, but his severe addiction to opium leads to financial ruin in Scotland. Believing he can discover a magical elixir of life by exploiting the knowledge of enslaved and Indigenous people, he brings Isobel to America, only to abandon her in Salem while he pursues his own schemes.

Mercy is a Black seamstress who lives in a small cottage near Isobel in Salem. She is deeply practical and fiercely protective of her family, possessing an acute understanding of the systemic prejudice in America. While she is initially guarded around the newly arrived Scottish immigrant, she slowly warms to Isobel, bonding over their shared talent for needlework.

William Darling is the capable and fair captain of the New Harmony. He forms a bond with Isobel during the Atlantic crossing after she nurses him through a life-threatening fever. He appreciates needlework himself, finding peace in stitching, and provides Isobel with valuable silk threads and practical advice for surviving in Salem.

Isobel Gowdie is a real historical figure and the direct ancestress of Isobel Gamble. Her story is chronicled in the novel's interludes, detailing her persecution by a corrupt reverend and her subsequent trial for witchcraft. Her legacy looms large over the Gamble family, serving as both a cautionary tale and a source of quiet strength for her descendants.

Zeke is Mercy's cousin and a Black wharf runner in Salem. He is intimately familiar with the town's geography and social dynamics, frequently assisting his family and neighbors with practical tasks. He works hard to protect his community while maintaining a careful distance from the white residents of the town.

Nell is an energetic young Irish housemaid working for the wealthy Silas family in Salem. As a fellow recent immigrant, she understands the prejudice directed at newcomers and quickly befriends Isobel. She leverages her position to help Isobel secure lucrative, secret sewing work for her employers.

Widow Higgins is an older Salem resident who acts as the Gambles' landlord. She is a former midwife whose practice suffered due to town gossip and prejudice. She maintains a sharp awareness of Salem's secrets and closely monitors the activities of her tenants, offering Isobel blunt advice about attending church and finding work.

Felicity Adams is the proprietress of a prominent dress shop in Salem. She descends from victims of the Salem Witch Trials but has carved out a respectable, if ruthless, business. She hires Isobel but exploits her desperation, paying her a meager wage while selling her finely embroidered gloves at a steep markup.

Abigail is a local stitcher who works alongside Isobel. She is highly attuned to the gossip and social dynamics of Salem, frequently sharing rumors about prominent families like the Hathornes. While sometimes nosy, she provides Isobel with vital information about the town's history and prejudices.

Charlotte is the daughter of the wealthy Silas family, whose fortune is tied to the slave trade and sugar plantations. She is pregnant out of wedlock, prompting her family to hastily arrange a wedding and hire Isobel to discreetly alter her bridal clothes.

Mrs. Silas is the matriarch of a wealthy Salem family that profits from enslaved labor. She is highly conscious of her family's social standing and fiercely protective of their reputation. She hires Isobel to perform secret alterations for her daughter, leveraging her wealth to demand absolute discretion.

Margaret MacAllister is a cautious woman living in Glasgow who deeply fears the legacy of witchcraft accusations in her family. Recognizing her daughter's synesthesia, she strictly teaches Isobel to hide her colorful perceptions to protect her from persecution, passing down the family tradition of hiding a scarlet letter in their embroidery.

Isobel's father is a practical Scottish man who believes a woman should focus on marrying well rather than harboring grand ambitions. Despite his traditional views, he demonstrates deep love for his daughter by paying off Edward's debts to rescue the couple from the poorhouse, slipping Isobel a secret bag of coins before she sails.

Ivy is a young Black girl living in Salem. She helps her family by delivering goat's milk and eggs to the neighbors. She is curious and friendly, representing the innocence that Mercy and Zeke work so hard to protect.

Abraham is a young Black boy living with Mercy. He assists his family with daily chores and navigating the social boundaries of Salem, growing up under the constant threat of the era's prejudiced laws.

John Hathorne is a historical figure featured in the novel's interludes. As a judge during the 17th-century Salem Witch Trials, his harsh condemnations and refusal to apologize for the executions create a dark legacy that hangs over his descendant, Nat, centuries later.

Ingo is a Black sailor and the first mate aboard the New Harmony. He is deeply loyal to Captain Darling and plays a crucial role in maintaining order on the ship, demonstrating calm competence during medical emergencies at sea.

Auntie Aileen is Isobel's relative and a skilled dressmaker in Scotland. She recognizes Isobel's connection to their ancestress, Isobel Gowdie, and likens the art of dressmaking to the act of secret-keeping, imparting early lessons about women's hidden power.