Charlie is a 13-year-old boy from a working-class family in London’s East End who drops out of school to secretly operate as a thief. He lives with his grandmother and struggles to ease the financial burden on their household. He maintains strict personal rules about stealing only from the wealthy and avoiding those in need. A past bombing that destroyed his school and killed his mother leaves him with severe, unacknowledged anxiety.

Molly is a 15-year-old from the wealthy Chelsea neighborhood who recently returns to London after working as an auxiliary nurse in the country. Her sheltered upbringing contrasts with the devastated city she discovers upon coming home. She arrives to an empty house, finding her parents missing and relying on her old nanny for answers. She possesses advanced medical knowledge but frequently faces dismissal due to her age and gender.

Ignatius is the tall, thin proprietor of The Book Keep and a volunteer air raid warden. A former schoolteacher and trained mathematician, he possesses a brilliant mind for puzzles and codes. He lives alone, grieving the loss of his wife and preserving her eclectic bookstore exactly as she left it. He projects a kind, paternal demeanor toward local children while conducting secretive meetings in the back of his shop.

Gran is Charlie's maternal grandmother and his sole remaining family member. She works long hours at a bakery to support them both, utilizing their meager ration books to stretch meals as far as possible. Highly intelligent and self-educated, she harbors a deep mistrust of the government's handling of the food supply. She remains unaware of Charlie's thievery, believing he still attends school.

Lonzo is an orphaned teenager living on the streets after running away from an abusive state-run facility. He survives by committing petty crimes alongside his closest companion, Eddie. He acts tough and frequently threatens others to get his way, masking a deep fear of being entirely alone in the war-torn city.

Eddie is a quiet, reserved orphan who lives in abandoned buildings with Lonzo. He possesses a talent for picking locks and crafts tools for breaking into buildings. He acts as the calmer, more practical half of his partnership with Lonzo.

Henry is Molly's father, a man who purportedly works long hours at the Ministry of Food. He disappears shortly before Molly's return to London, leaving behind only a brief, apologetic note. His absence causes immense anxiety for his daughter, who attempts to track him down through the deserted city streets.

Eloise is Molly's mother, who was separated from her daughter at the outbreak of the war. She suffers from severe mental health issues following a traumatic event in a bomb shelter. Her condition requires long-term confinement in a facility in Cornwall, entirely removing her from her daughter's daily life.

Imogen is Ignatius's late wife, who inherited The Book Keep from her well-connected father. She believed passionately in the power of books to comfort people during times of distress, insisting on opening the shop in a working-class neighborhood. She leaves behind an unfinished manuscript that Ignatius obsesses over in his grief.

Dr. Stephens is the head doctor at the Beneficial Institute in Cornwall, a psychiatric hospital treating victims of severe trauma. He writes regular updates regarding patient conditions and attempts to manage the overwhelming influx of psychiatric casualties caused by the war.

Dr. Foyle is a physician at the Beneficial Institute who administers extreme, experimental surgical treatments to patients. He struggles with the lack of effective psychiatric care available, a frustration born from his own father's trauma after World War I.

Detective Inspector Willoughby is a severe, cruel policeman operating in London. Jaded by the horrors of the city, he frequently abuses his authority and employs physical violence against vulnerable suspects. He pursues petty criminals with extreme aggression, showing little regard for the law he supposedly upholds.

Mrs. Pride is the long-serving nanny and housekeeper for the Wakefield family. She struggles to maintain the large Chelsea home alone after the parents disappear. She acts evasive about the family's true situation, trying to protect Molly from the harsh realities of her parents' absences.

Desdemona is the proprietress of a shop neighboring The Book Keep. She keeps a close, suspicious eye on Ignatius's activities and visitors. She considers it her civic duty to watch for unusual behavior and regularly probes visitors for information.

Cedric is a short, mysterious man who conducts secretive, late-night exchanges with Ignatius. He uses hollowed-out books and cryptographic devices to pass information, operating entirely in the shadows of the war-torn city.