Simon is a British and Jewish Royal Air Force pilot who is shot down outside Berlin. Michael discovers him and he becomes a significant figure in Michael's life, helping him understand the personal stakes of the war and encouraging him to overcome his fear of heights. He plays a crucial role in the novel, driving the narrative forward by involving Michael in the effort to recover the plans for the Germans' Projekt 1065.

Fritz is Michael's classmate, initially introduced as a bully's target. Michael befriends and mentors him in self-defense. As his confidence grows, Fritz transforms into one of the bullies, representing how Nazi ideology corrupts young minds. His character arc is pivotal in highlighting the impact of propaganda on impressionable youth.

Davin is Michael's father and the Irish ambassador to Germany. A pragmatic man dedicated to aiding the Allies, he is protective of Michael and often argues with his wife about the risks involved in their espionage activities. He seeks to ensure his family's safety amid increasing danger.

Megan is Michael's mother, a trained espionage agent working secretly for the Allies. She encourages Michael's involvement in spying missions, believing in the underestimated potential of women and children as spies. Her character represents the clandestine side of the family's efforts against the Nazis.

Melcher is Michael's teacher, known for his gruff demeanor and numerous titles. Though he appears loyal to the Nazis, he secretly disapproves of their regime. His eventual fate underscores the harsh reality for those who quietly resist the Nazi regime.

Though a historical figure, Hitler functions as a character in the novel, symbolizing the Nazi regime and its hypocritical and oppressive nature. His charismatic and intimidating presence influences the characters' experiences and highlights the era's complex social dynamics.