Janie is a 14-year-old girl who idolizes Katharine Hepburn and longs for independence and confidence. Uprooted from Los Angeles due to the political persecution of the Second Red Scare, she struggles to adapt to her new life in England. She is highly observant, intelligent, and increasingly willing to believe in scientific possibilities that defy strict logic.

Benjamin is the defiant, brave son of a local apothecary. Having survived the London Blitz, he is skeptical of superficial safety measures and desires a thrilling life as an international spy rather than following in his father's footsteps. He is highly intelligent and grounded in strict physical laws, initially resisting the existence of magic.

Marcus is a skilled London apothecary who comes from a long line of healers entrusted with an ancient text called the Pharmacopoeia. Devoted to making the world safer after losing his wife in World War II, he conducts highly secretive experiments that blend chemistry with ancient mysticism.

Mr. Danby is a young, handsome Latin teacher at St. Beden's who formerly served as a Royal Air Force pilot and was a prisoner of war. He treats his students with adult respect, making him a quick favorite of Janie's. He takes a keen interest in his students' lives outside the classroom and maintains a worldly, intellectual demeanor.

Vili is a Hungarian physicist assisting Marcus Burrows with top-secret experiments. He is a brilliant intellectual who balances a sardonic, jaded demeanor with an endless willingness to be pleased. He possesses a unique understanding of time and the consequences of tampering with natural laws.

Jin Lo is a highly skilled Chinese chemist who originally apprenticed in Shanghai. She corresponds with Marcus Burrows and travels to London to assist in his scientific efforts. She is resourceful, incredibly capable under pressure, and masterful at creating specialized chemical reactions and elixirs.

Pip is a nimble, quick-witted pickpocket and housebreaker from London's East End. Despite his youthful criminal habits, he forms a fast bond with Janie and Benjamin at a juvenile detention center. He is incredibly loyal, excellent at chess, and uses his street smarts to help his new friends.

Sarah is a wealthy, healthy, and beautiful student at St. Beden's who appears to have a perfect life. Initially perceived as slightly snobbish due to her pristine appearance, she demonstrates a genuinely helpful and generous nature when her classmates require assistance.

Leonid is an accountant at the Soviet embassy and a secretive figure operating in London. Recognizable by his wooden leg, he maneuvers through dangerous political waters while desperately trying to protect his wife and daughter who remain in Russia.

Sergei is a quiet student at St. Beden's grammar school who radiates a palpable sense of loneliness. He aids Benjamin and Janie when they are in trouble, deeply affected by the precarious political position his family occupies.

Marjorie is Janie's mother, an entertainment industry writer who flees the United States to avoid testifying before the House Un-American Activities Committee. She takes a job writing for the BBC under an assumed name, trying to maintain a normal life for her daughter.

Davis is Janie's father, a writer forced into exile during the Second Red Scare. He tries to shield his daughter from the harsh realities of their political persecution while adapting to life in a bomb-scarred London.

The gardener is an elderly, steady man whose hands resemble the rough bark of the trees he cares for. He possesses deep knowledge of the Pharmacopoeia and encourages the children to keep an open mind about the possibilities of ancient medicine.

A menacing man with a distinctive scar down one cheek. He operates as a driver and enforcer for the East German secret police, ruthlessly pursuing the Pharmacopoeia and anyone connected to it.

Detective Montclair is a harsh Scotland Yard detective who investigates the Scotts for communist ties and aggressively pursues the children when they run afoul of the authorities.

A police officer who works alongside Detective Montclair, assisting in the physical capture and transport of the children to juvenile detention.

Mrs. Parrish is the Scotts' landlady in London. She is visibly displeased about renting her flat to Americans and harbors lingering anxieties and strict rules stemming from the war.

Olivia is a boss at the BBC who oversees the Scotts' writing. She treats Janie with respect and engaging wit, helping the young girl feel less homesick in her new environment.

Captain Norberg is the commanding officer of an icebreaker ship headed into dangerous waters. He is cooperative with the scientists' mission and demonstrates deep loyalty to his crew.

Sakharov is a brilliant, young Soviet scientist responsible for designing devastating new bomb technology, drawing the intense interest of both military forces and rival scientists.

Hirra is a Norwegian man living in a smoky coastal hut who provides crucial, life-saving sanctuary to the children when they fall into freezing waters.