Christopher is a fourteen-year-old orphan who formerly lived at the Cripplegate orphanage before being chosen for a specialized trade. He is currently serving as an apprentice to Master Benedict Blackthorn, learning the highly technical and secretive skills of an apothecary. Though he is exceptionally clever and quick to decode complex ciphers, his natural curiosity often leads him to act impetuously. He is deeply grateful for his apprenticeship and fiercely protective of those who show him kindness.

Master Benedict is a highly respected and successful apothecary working in 17th-century London. He operates a well-stocked shop and possesses deep knowledge of various remedies, chemicals, and complex ciphers. He treats his young apprentice with a mixture of strict discipline and genuine fairness, aiming to provide a thorough education. He harbors concerns about recent murders in the apothecary community and fiercely protects his trade secrets.

Tom is the fourteen-year-old son of a local baker and Christopher's closest companion. Physically larger than his friend, he is naturally cautious and frequently worries about the trouble Christopher's experiments might cause. Despite his hesitation and fear of authorities, he remains consistently loyal and repeatedly joins Christopher in his unpredictable endeavors. He comes from a large, bustling family that occasionally provides Christopher with a sense of normal domestic life.

Oswyn is a prominent member of the Apothecaries' Guild Council and a strict Puritan, a dangerous religious affiliation under the current monarchy. He is responsible for recognizing Christopher's potential at the Cripplegate orphanage and securing his apprenticeship. He maintains significant influence within the Guild and represents the formal authority structure governing London's apothecaries.

Lord Ashcombe serves as the Warden for King Charles, acting as the crown's direct enforcer on the streets of London. He is an imposing, authoritative figure leading an active investigation into a string of apothecary murders. He represents the absolute power of the monarchy and demands strict compliance from both commoners and Guild members alike.

Stubb is a competing apothecary who aggressively pursues greater wealth and status in London. He has a history of accusing others of stealing his recipes and holds a long-standing desire to acquire Master Benedict's shop. He uses his position as a Guild member to belittle those beneath him in the social hierarchy, acting with consistent cruelty toward apprentices.

Hugh is a trained apothecary who formerly served as an apprentice under Master Benedict. He currently works in the offices of Dr. Nicholas Lange. He maintains a close connection with his former master, frequently discussing the rising dangers and tensions within the apothecary community.

Isaac operates a bookshop overflowing with texts on numerous subjects. He supplies Master Benedict with specialized volumes, facilitating Christopher's broad education. He holds deep knowledge regarding esoteric symbols and ancient texts.

Sir Edward is a high-ranking member of the Apothecaries' Guild Council. He is primarily concerned with Guild protocol, legal ownership, and maintaining order within the profession during times of crisis.

Valentine is a master apothecary who sits on the Guild Council alongside Sir Edward and Oswyn. He participates in the official oversight of apprenticeships and shop ownership disputes.

Wat is an older apprentice working in the London apothecary community. He frequently acts on behalf of senior Guild members and displays a hostile attitude toward younger apprentices like Christopher.

Known to Christopher only by his imposing size, the Elephant is a physically massive apprentice in the Apothecaries' Guild. He uses his bulk to intimidate others and serves as a heavy-handed assistant to his superiors.

Martin is an apprentice who guards stairways and offices within the Guild Hall. He participates in coordinated efforts to corner or intimidate younger apprentices under the orders of full Guild members.

William is Tom's father, a hardworking local baker striving to support a large family. He is highly concerned with his family's reputation and financial stability, making him deeply suspicious of Christopher's chaotic influence on his son.

Mary is Tom's mother, a busy but compassionate woman running a bakery household. She provides Christopher with leftover clothes and a rare sense of maternal care, softening her husband's strict stance.

Cecily is Tom's oldest sister. She helps run the family bakery and enjoys asserting her authority over her younger brother and his friends during household chores.

Molly is Tom's little sister. Young and affectionate, she provides innocent comfort to Christopher during times of distress and acts as a messenger for her brother.

Dr. Parrett is a bedraggled man living in the ruins of a home known as the Black House, which burned down the previous year. He suffers from immense grief, frequently speaking of his deceased son as if the boy were still alive.

James is the deceased son of Dr. Parrett. He tragically died in a house fire the previous year, though his grieving father frequently speaks of him as if he were still alive.

Sally is a young girl currently living at the Cripplegate orphanage, where Christopher formerly resided. She faces an uncertain future on the streets once she ages out of the institution.

Bridget is a pigeon that provides saltpeter for the apothecary shop. She shares a special bond with Christopher, who pets and feeds her, offering him simple companionship amidst his complex duties.