Nella is a 41-year-old apothecary operating a concealed shop in 18th-century London. While she continues to sell the healing remedies her late mother specialized in, she also secretly dispenses fatal poisons to women seeking freedom from abusive men. Her clandestine work takes a severe physical toll on her body, manifesting as a mysterious illness and chronic joint pain. She operates by a strict personal code that dictates her poisons must never harm another woman and every transaction must be meticulously recorded in her calfskin register.

Eliza is a 12-year-old housemaid working at the Amwell estate. Originally from the farming village of Swindon, she moved to London for domestic work and learned to read under her mistress's tutelage. She demonstrates remarkable resolve by volunteering to serve poisoned eggs to her predatory master. She holds an intense curiosity about the natural world and readily believes in the power of magic.

Caroline is a 32-year-old farm administrator from Ohio visiting London on a solo trip. Once an aspiring historian who dreamed of attending Cambridge, she abandoned her academic pursuits to support her husband's corporate career. Her discovery of his infidelity prompts her to travel alone to reevaluate her life and marriage. While exploring the River Thames, she finds an old apothecary vial that reignites her passion for historical research.

Lady Clarence is the wealthy mistress of the Clarence estate in Carter Lane. Accustomed to wielding absolute influence, she exhibits an entitled demeanor that clashes with Nella's usual working-class clientele. She harbors deep frustration over her inability to conceive a child and seeks a toxic solution to eliminate her husband's mistress to clear her path to domestic stability.

James is Caroline's husband. He prioritizes corporate ambition and practical stability over Caroline's academic dreams. He shatters their decade-long marriage by engaging in a months-long affair with a work colleague. Despite Caroline's express desire for space, he flies to London uninvited to push for a reconciliation.

Gaynor is an employee at the British Library who works at the Maps Enquiry Desk. She brings warmth and professional expertise to Caroline's amateur historical investigation. Through her access to archival maps and manuscript databases, she provides the crucial resources needed to trace the origins of the mysterious blue vial.

Mrs. Amwell is the mistress of the Amwell estate and Eliza's employer. She suffers from a chronic tremor in her hands, leading her to rely on Eliza to write her correspondence. While she acts gently toward Eliza and teaches her to read, she also orchestrates the plot to poison her husband after witnessing his predatory behavior toward the young maid.

Mr. Amwell is the middle-aged master of a household in Warwick Lane. He abuses his position of power to prey on the young female servants under his roof. He uses alcohol to incapacitate his staff and subjects them to unwanted sexual advances, creating an atmosphere of quiet terror in the domestic space.

Lord Clarence is the wealthy master of the Clarence estate. He conducts an ongoing affair with his younger cousin, Miss Berkwell, right under his wife's nose. His infidelities drive the central conflict of the historical timeline as his wife seeks a permanent solution to his wandering eye.

Miss Berkwell is a petite young woman who is both the cousin and the mistress of Lord Clarence. Her affair with the lord of the estate draws the severe ire of Lady Clarence, making her the unwitting target of a deadly plot.

Frederick is a young, dark-haired meat merchant from Nella's past. He entered into a passionate romance with a young Nella shortly after her mother's death, offering her comfort during her grief. His severe betrayal regarding his marital status and his actions regarding Nella's pregnancy permanently altered the trajectory of her life.

Johanna was a housemaid at the Amwell estate just a few years older than Eliza. According to the household staff, she was impregnated by Mr. Amwell and suffered a tragic fate. Her memory lingers in the house, fueling Eliza's growing belief in restless spirits.

Tom is a teenaged shopkeeper working in a store that specializes in books on the magical arts. He shares a personal connection to the shop's inventory, believing a spell from one of the books saved his life at birth. He quickly develops a friendly rapport with Eliza, offering her a book and encouraging her curiosity.

Bachelor Alf is a boisterous, gray-haired British gentleman who leads mudlarking expeditions along the River Thames. He scrounges the riverbanks for historical artifacts and takes great joy in his hobby. He serves as a catalyst for Caroline's historical investigation by helping her identify the blue vial.

Rissa is Frederick's wife. She approaches Nella's shop after growing sick of her husband's constant cheating. Her request for a lethal solution marks a pivotal moment in the history of the apothecary shop at Back Alley.

Sally is the cook and kitchen maid at the Amwell house. She acts as an early guide for Eliza when the young girl first arrives to work in the kitchen, teaching her practical skills and sharing the dark history of the estate's former servants.