Alex is an 11-year-old Jewish boy living in an unnamed city in occupied Poland during World War II. Accustomed to hiding to avoid "selection," he must survive in a dangerous environment independently after becoming separated from his father. He relies on his sharp memory, street smarts, and an old Beretta pistol to stay alive in a ruined building on Bird Street.

Alex's Father is a rope factory worker who shares a bunker with his son and the Gryns to avoid German selection. He is a pragmatic but idealistic man who believes people should not be judged by ethnicity or religion. He actively prepares Alex for survival by teaching him self-defense, weapon use, and the necessity of having multiple escape routes.

Boruch is the storeroom manager at the rope factory where Alex's father works. He is more pessimistic than Alex's father, believing the terrifying rumors about the German death camps. He teaches Alex how to tie sturdy knots and steps up to protect him during the factory selection.

Bolek is a Polish man living in the Polish quarter and a member of the underground resistance. He takes a liking to Alex during a chance encounter and offers him an address to reach out for help. He frequently takes great personal risks to aid Jewish people attempting to escape the ghetto.

Stashya is a young girl living in the Polish quarter outside the ghetto wall. Initially known to Alex only as the "homework girl," she eventually meets him when he ventures outside the ghetto. She quickly develops a sweet, innocent romance with him and bravely learns to communicate with him through secret visual codes.

Henryk is a Jewish resistance fighter participating in the uprising in Ghetto A. After sustaining a gunshot wound, he is forced to hide in Alex's shelter while his companion Freddy continues the fight. He shares strong Zionist beliefs and tells Alex about his hopes for a persecution-free homeland in Palestine.

Freddy is a Jewish resistance fighter on his way to join the uprising in Ghetto A. He bandages Henryk's wound and stays just long enough to ensure his friend is safe before leaving to fight against the German forces.

Dr. Polaswski is a physician living in the Polish quarter who secretly treats members of the Polish underground. He answers to a secret code and risks his life to enter the ghetto to perform emergency surgery on Henryk.

Yanek is a neighborhood bully living in the Polish quarter. He exhibits strong antisemitic prejudices and threatens Alex's safety by confronting him and accusing him of being Jewish.

Marek is a Jewish father hiding in the lofts with his family. Despite their shared predicament, he is initially hostile toward Alex, slapping him and attempting to hoard the discovered food until his family intervenes.

Martha is a young Jewish girl hiding with her family in the interconnected lofts of the ghetto. She begs her father to share food with Alex and later receives Alex's help when a man harasses her near the rope factory.

Alex's Mother went missing while visiting a friend in another ghetto shortly before the main events of the story. Through Alex's memories, she is depicted as a realist and a Zionist who longed for a home in Palestine where her family could be free from persecution.

Snow is Alex's pet white mouse. Trained to search for crumbs when Alex whistles, Snow provides crucial companionship and emotional comfort to the isolated boy during his long months of hiding.

Wlodek is a Polish boy who plays in the park outside the ghetto walls. He acts as a captain during a snowball fight and eventually defends Alex from teasing, providing Alex with a brief taste of normal childhood interaction.

Pani Gryn shares the original apartment and bunker with Alex and his father. When Alex returns to the apartment after escaping the selection, she and her family have taken all of his father's stashed food.

Yossi is the young son in the Gryn family, who shares the initial apartment with Alex and his father. Alex later finds and uses his abandoned backpack for gathering supplies.