Marie-Laure is a blind teenager from Paris who learns to move through the world under the patient guidance of her father. She holds a deep love for the natural world, particularly marine biology, and possesses a vivid imagination fueled by reading braille adventure novels. Forced to flee Paris due to the advancing war, she relies on her resourcefulness and sharp hearing to survive in the walled coastal city of Saint-Malo.

Werner is a highly intelligent orphan with white-blond hair from the German coal-mining town of Zollverein. Desperate to avoid working in the local mines where his father died, he hones a natural talent for building and repairing radios. His technical skills offer him a path out of poverty via a state-sponsored military school, forcing him to make increasingly heavy moral compromises to secure his future.

Daniel is the master locksmith at the National Natural History Museum in Paris and a devoted single father. He carefully constructs wooden scale models of their neighborhoods and carves intricate puzzle boxes to help his blind daughter develop spatial awareness. When the war breaks out, the museum directors entrust him with an important secret, forcing him to flee the city to protect his child.

Jutta is Werner's younger sister, an artistic and strong-willed girl raised in a Protestant orphanage in Germany. She shares her brother's fascination with science and long-distance radio broadcasts. She holds a firm moral compass, questioning state propaganda and expressing deep concerns about the political changes sweeping across their country.

Etienne is Marie-Laure's great-uncle, an educated and sensitive man living in the coastal city of Saint-Malo. Traumatized by his experiences in the First World War, he suffers from severe agoraphobia and rarely leaves his home. He finds solace in his extensive collection of radios, scientific equipment, and books, hiding away from a world he finds terrifying.

Madame Manec is the longtime housekeeper for Etienne in Saint-Malo. She is a pragmatic, energetic, and deeply caring woman who feeds the needy and maintains order in the household. Despite her advanced age, she possesses a fierce spirit and refuses to remain passive in the face of the encroaching military occupation, organizing small acts of rebellion in her community.

Reinhardt von Rumpel is a German military officer tasked with finding and seizing valuable antiques, jewels, and art for the regime. He is a determined, methodical gemologist who becomes obsessed with locating a legendary diamond known as the Sea of Flames. His relentless hunt is driven by a desperate personal need and unwavering dedication to his mission.

Frederick is a student from a wealthy, privileged Berlin family who attends the state-sponsored military school with Werner. He is a gentle dreamer with a passion for ornithology and the natural world. Lacking the physical ruthlessness expected of the cadets, he struggles to survive in the brutal, hyper-masculine environment of the institution.

Frank is an older cadet at Schulpforta, known to the other boys as "the Giant" due to his massive physical size. He serves as an enforcer of orders within the school and later as a soldier on the front lines. Despite his imposing appearance and capacity for violence, he shows surprising gentleness toward Werner, acting as an unofficial protector.

Frau Elena is a Protestant nun from Alsace who manages the orphanage where Werner and Jutta grow up. She is a kind, patient, and resourceful caretaker who encourages the children's learning. As the political climate in Germany shifts, she tries to protect the children while hiding her French background and accent to avoid drawing suspicion.

Dr. Hauptmann is a highly intelligent science instructor at the Schulpforta academy. He recognizes Werner's exceptional aptitude for mathematics and engineering, bringing the boy into his private laboratory to work on complex directional radio transceivers. He values scientific progress but ties it entirely to the success of the state military apparatus.

Walter is an engineer serving in the same military unit as Werner and Volkheimer. When the American bombers strike Saint-Malo, he is caught in the subterranean collapse of their hotel headquarters, sustaining severe injuries in the dark.

Herr Siedler is a wealthy state official living in Zollverein. His house is filled with forbidden luxuries, including a powerful shortwave radio. After Werner successfully repairs the machine, Siedler recognizes the boy's talent and offers to write a recommendation letter to help him enter a prestigious military school.

Hubert is an unhoused veteran of the First World War living in Saint-Malo. He wears the physical scars of his service, having suffered a severe facial disfigurement. He shares local history with Marie-Laure, eventually trusting her with access to a hidden grotto under the ancient city walls.

Bastian is the field commandant at Werner's school. He is a harsh, relentless instructor who enforces the brutal physical and psychological conditioning of the boys, intentionally pitting them against one another to identify the weakest among them.

Big Claude is a local perfumer in Saint-Malo who takes advantage of the wartime occupation to enrich himself. He collaborates with the occupying authorities and keeps a watchful eye on his neighbors, prioritizing his own comfort and wealth over loyalty to his community.

Dr. Geffard is a mollusk expert working at the National Natural History Museum in Paris. He befriends Marie-Laure when she visits her father at work, eagerly answering her questions and sharing his extensive knowledge of the ocean and evolutionary history.

Madame Ruelle is the baker's wife in Saint-Malo. She is an active participant in the local community and works quietly behind the scenes to assist the town's resistance efforts by baking covert messages into everyday loaves of bread.

Henri is Etienne's late brother and Marie-Laure's grandfather. Though he died years prior in the First World War, his legacy lives on through the children's science broadcasts he created with Etienne. His voice reaches across borders, profoundly impacting children who listen to his lectures on light and science.

Dupont is a Parisian lapidary and jeweler held prisoner by the occupying forces. He possesses specialized knowledge about the intricate replicas made of the Sea of Flames diamond to hide the original stone, and he is pressured to share this information with the military.