Max is an 11-year-old Jewish boy from Germany who possesses a genius-level aptitude for building and mending radios from scrap parts. Placed with the wealthy Montagu family in London, he suffers from deep homesickness and projects a precocious need to protect his vulnerable parents. He responds to his new, sometimes hostile environment by engineering elaborate pranks, eventually plotting to volunteer as a British spy so he can return home.

Ewen is a high-ranking officer in Naval Intelligence and the younger brother of Mr. Montagu. He approaches the war with a pragmatic, institutional mindset, believing that creative fictions and deception are necessary tools for statecraft. He is the first adult to recognize Max's strategic brilliance and recruits him into the espionage program.

Ivor is the eccentric, communist younger brother of the Montagu family who frequently clashes with Ewen over politics. He quickly befriends Max through intense basement table tennis matches. He acts as a critical voice against British imperialism, explicitly warning Max about how intelligence agencies exploit people as disposable resources.

Colonel Roberts is the chief interrogator at Camp 020, an intelligence prison in Richmond. He wears a metal monocle and relies on brutal psychological tactics to break suspects. He deliberately utilizes racist and antisemitic slurs to test the loyalty and resilience of his prisoners.

Stein is a tiny, immortal dybbuk who hitches a ride out of Germany alongside Max. Identifying as a spirit of the Jewish people, he claims his primary purpose is to heckle and annoy humans. Visible only to Max, he serves as a physical manifestation of the boy's Jewish identity and internal emotional struggles.

Berg is a German kobold, a spirit tied to the physical land of Germany, who escapes the Nazi regime by traveling with Max. He is prone to minor acts of sabotage, like tying shoelaces together or chewing through twine. Alongside Stein, he provides a running, bickering commentary that reflects Max's conflicted national identity.

Jean is a 19-year-old operative for Naval Intelligence who acts as Max's espionage handler, or "Mother." She is highly athletic and possesses a sharp understanding of psychology. She pushes Max through rigorous physical and mental challenges, teaching him to embrace risk and learn from failure.

Lord Rothschild is a wealthy, brilliant British Jewish aristocrat who has turned his sprawling Tring Park estate into a secret training facility. He acts as an explosives expert for the intelligence service while maintaining a welcoming demeanor. He offers Max profound insights into the precarious social position of Jewish people in Europe.

Sergeant Thompson is the Head of Security for the Military Police at the training estate. Originally from Trinidad, he harbors deep resentment toward British colonial rule but joined the army to follow his wife to England. He shares his history with Max, validating the idea that doing anything for loved ones is morally sound.

Mr. Montagu is the wealthy patriarch of the household where Max is placed. He provides a comfortable environment and leads his family in Jewish traditions, but he displays a strict temper when dealing with schoolyard misbehavior.

Mrs. Montagu is the matriarch of the household. She is warm and welcoming to her new foster child, attempting to provide stability and comfort by leading the family's Shabbat observances.

David is the older Montagu son and attends St. West's School in the same class as Max. He attempts to explain the institution's strange slang and warns Max to hide his Jewish heritage from the hostile student body.

Anthony is the younger son of the Montagu family. He looks up to Max with deep admiration, especially after the older boy steps in to physically threaten the bullies tormenting him at school.

Harold is the only Indian student at St. West's School, making him a frequent target of the administration's racism. He advises Max to keep his head down, pointing out that Max's white skin gives him a distinct advantage in surviving the school.

Master Yarrow is a teacher who champions imperialist values and actively encourages the bullying of marginalized students. He singles Max out for his German background, using obscure English place names to humiliate him.

Circuitt is a cruel ringleader among the students at St. West's School. He targets Max and young Anthony with antisemitic insults and uses the violent rugby trials as an excuse to assault other boys.

Lieutenant Chumley is a towering espionage instructor who tasks Max with his central training objective: placing a correctly spelled note in a highly monitored dead letter box.

Admiral Godfrey is the head of Naval Intelligence and Ewen's superior. He is heavily focused on the broad strokes of the war, viewing field operatives—including child recruits—as expendable assets.

Dr. Brown is a pompous psychological assessor tasked with determining Max's fitness for his mission. He attempts to block the boy's deployment until Max utilizes secret intelligence to expose his past political associations.

Major Jameson is a cheerful commando assigned to drop behind enemy lines with Max. He is tasked with executing a tandem parachute jump to ensure the boy reaches German soil.