Lidia is a 12-year-old girl living in Warsaw who loves playing the piano. Raised in a wealthy household, she suddenly faces the brutal realities of the Nazi occupation of Poland. She is fiercely strong-willed and struggles to sit by quietly as the Jewish population is moved into ghettos and the city is taken over, prompting her to seek out the underground resistance.

Janina is Lidia and Ryszard's mother. Accustomed to a comfortable, wealthy life before the war, she initially tries to ignore the changing reality of the Nazi occupation. She struggles to express affection to Lidia because she carries deep, unaddressed grief from the loss of her toddler daughter, Krystyna. Her primary focus is keeping her remaining family members out of the conflict.

Ryszard is Lidia's older brother. He quickly realizes the severity of the German invasion and begins working construction for the occupying forces as a cover for his resistance activities. He distributes underground newspapers and gathers intelligence, all while strictly commanding Lidia to stay away from the dangerous underground movement.

Officer Schubert is a cruel German soldier who patrols the streets of Warsaw. He enforces the occupation's brutal laws, frequently stopping Lidia and Ryszard to humiliate them, strike them, or search them for contraband.

Maryna is a young girl exactly Lidia's age who has also lost her father to the war. Resourceful and clever, she figures out a way to drop food into the Jewish ghetto using a rooftop drainpipe. She introduces Lidia to a secret underground school when public education is outlawed by the Germans.

Doda is the Durr family's former maid. As a Jewish woman who previously fled Nazi Germany, she fully understands the threat the occupation poses and warns the family about what is coming. When the Durrs lose their home and wealth, she coordinates with other former employees to secure a small apartment for them.

Papa is Lidia's father and a veteran of the Great War. When the invasion begins, he abandons his comfortable life as a wealthy professor, trades his automobile for a safer horse-drawn wagon to get his family to Warsaw, and immediately reenlists to fight the approaching armies.

Stefan is a young resistance fighter who remembers Lidia playing piano at an orphanage years earlier. He works closely with the underground movement and acts as a gatekeeper, testing Lidia's ability to count soldiers and gather intelligence before allowing her to officially join.

Halama is Maryna's uncle and an established member of the Polish resistance. He tests Lidia's convictions during a dinner conversation by questioning the value of helping the Jewish population. Impressed by her moral arguments, he provides her with the address to join the movement.

Drill is a young resistance member who evaluates Lidia's motivations for joining the fight. He questions her fears and ultimately accepts her into the group, assigning her dangerous tasks like delivering papers and hiding grenades.

Arrow is a resistance leader who coordinates dangerous missions, such as rescuing stolen Polish children from German transport trucks. He issues commands to the younger members of the resistance, evaluating their courage under fire.

Gray is an older woman involved in the resistance. She meets Lidia during a trolley ride where Lidia is avoiding a German officer. Gray engages Lidia in a conversation about music, comparing life to a symphony where every note matters.

Weasel is a young, silent Jewish boy whom Lidia takes in during the uprising. Despite his young age and initial silence, he proves incredibly helpful to the resistance by delivering mail within the safe zone and performing highly dangerous tasks in the streets.

Bubbe is Doda's mother. As a Jewish refugee who fled Germany, she understands the tactics the Nazis use to isolate and destroy populations, and she warns the Durr family about the terrifying reality of the occupation.