Michael is the 13-year-old son of the Irish ambassador to Germany, living in Berlin in 1943. With his brown hair, brown eyes, and fluent German, he passes easily as a typical German boy, allowing him to infiltrate the Hitler Youth while secretly acting as a spy for the Allies. He harbors a deep hatred for bullies, stemming from his own experiences at an English school, and possesses a photographic memory that proves invaluable for gathering intelligence. He struggles with a severe phobia of heights.

Simon is a Jewish, British Royal Air Force pilot who is shot down while taking reconnaissance photos of a secret German aviation project. Hidden in the Irish embassy's secret room, he relies on the O'Shaunessey family for survival while recovering from his injuries. He maintains a lighthearted demeanor, frequently trading jokes with Michael about the English and Irish, and encourages the young boy to read banned books and face his fears.

Fritz is a scrawny, awkward 13-year-old boy who frequently becomes the target of bullies. Despite his physical weaknesses, he holds a fierce devotion to Nazi ideology and dreams of joining the elite SRD (junior Gestapo) and eventually the SS. He keeps a secret stash of banned British detective novels, revealing a more complex personality beneath his eagerness to fit into the ruthless Hitler Youth culture.

The dictator of Nazi Germany, Hitler acts as the ideological center of the regime Michael is fighting against. He uses his captivating rhetoric to groom young boys into fanatical soldiers, promising them glory while demanding absolute sacrifice. His physical appearance starkly contradicts the Aryan physical ideals he demands of the German people.

Davin is the official Irish ambassador to Germany and an undercover informant for the Allies. He maintains a careful diplomatic facade while using his position to gather and transmit German secrets. As a father, he is highly protective and frequently expresses anxiety about Michael taking on dangerous espionage tasks, preferring to keep his son out of the direct line of fire.

Megan is a highly trained operative for Irish Intelligence who has been working as a spy since she was sixteen. Under the cover of an ambassador's wife, she runs an informant network in Berlin and orchestrates complex clandestine operations. Unlike her husband, she actively encourages Michael's involvement in espionage, teaching him practical spycraft and valuing the unique access children have to restricted spaces.

Melcher is an elderly former college professor and World War I veteran who has been reduced to teaching 13-year-old boys. He possesses a gruff demeanor and holds a deep, poorly concealed disdain for Nazi ideology. He subtly challenges Hitler's doctrines in his classroom, pointing out the hypocrisy of Nazi racial theories to a room full of impressionable, potentially dangerous Hitler Youth members.

Horst is the leader of Michael's junior Hitler Youth squad. Described by Michael as a donkey-faced bully, he eagerly enforces Nazi cruelty and strictly punishes any signs of weakness or compassion among the boys. He uses his authority to terrorize younger members, demanding absolute obedience to the party line.

Trumbauer is an SS officer whose primary job is to hunt down Jews hiding in Berlin. He is a menacing, observant figure who oversees special operations involving the elite Hitler Youth divisions. He recognizes ruthlessness in young recruits and actively recruits boys who demonstrate fanatical loyalty to the Nazi cause.

Lina is Fritz's ten-year-old sister. She frequently observes Michael with wide, saucer-like eyes when he visits the Brendler home. Strongly influenced by the environment around her, she fully absorbs the societal expectations placed on young German girls by the Nazi state.

Hendrik Goldsmit is a Dutch physicist who holds critical knowledge regarding the development of the atomic bomb. He is sought after by the Allies as part of an operation to recruit German and European scientists. His presence at a conference in neutral Switzerland makes him a prime target for Nazi operatives.

Max is a member of the elite SRD youth squad who speaks English, earning him a place on a special, secretive science team. He mans the antiaircraft guns alongside Michael and is quick to notice and challenge any suspicious behavior among his peers.

Ottmar is a zealous teenage member of the SRD chosen for a covert international mission. He is fully committed to the destructive goals of the Nazi party, showing zero remorse for the prospect of killing innocent civilians to achieve his squad's objectives.

Erhard is another teenage SRD member recruited for a covert assassination operation. Like his peers, he has fully absorbed Nazi propaganda and readily participates in violent operations under the guise of serving the Fatherland.