Petra is a twelve-year-old girl living in a small Mexican village during the early 20th century. Following the loss of her mother and the conscription of her father, she takes on the immense responsibility of providing for her younger siblings and grandmother. She chops and sells firewood, resisting expectations that dictate she should focus solely on domestic chores. Her ultimate aspiration is to learn to read and write so she can eventually become a teacher.

Amelia is Petra’s six-year-old younger sister. She assumes significant daily chores, including caring for her infant brother and assisting with cooking, leaving her little time to act like a typical child. Despite the family's difficult circumstances, she maintains a gentle spirit and forms strong attachments to the family's animals.

Abuelita is the elderly grandmother of Petra, Amelia, and Luisito, and the mother of their father. Having survived poverty and marginalization by adhering to established norms, she expects her granddaughters to follow a similar path of domestic duty. She frequently criticizes Petra's non-traditional ambitions, fearing that deviating from cultural expectations will put the girl in danger.

Papá is the father of Petra, Amelia, and Luisito. He is a man of integrity who dreams of a better life and encourages his daughters to pursue knowledge beyond domestic skills. Before being forced to join the Federal Army, he secures a promise from Petra to care for the family in his absence.

Mamá is the deceased mother of Petra, Amelia, and Luisito. She passed away while giving birth to Luisito about a year before the primary events of the story. Her children remember her as a joyful woman who worked to bring them comfort, and they continue to grieve her absence deeply.

Luisito is the youngest child in Petra's family. As an infant whose mother died during his birth, he relies entirely on his sisters and grandmother for survival. His vulnerability drives much of the family's urgency to find safety and medical care.

Adeline is a white American girl living in Mexico who meets Petra while taking refuge in a church. She quickly befriends Petra, offering her food and teaching her how to spell her name. Having lost her own older sister, Adeline eagerly forms a strong connection with Petra and invites her to travel safely to the United States.

Marietta is a highly respected captain in the Mexican rebel forces. Wearing pants and operating outside conventional gender roles, she provides a powerful model of independence for Petra. She acts with compassion, securing medical help for Luisito and later offering to train Petra as a soldier in her camp.

Pablo is Petra's cousin who is eligible to fight in the war. The Federales target Petra's family hut specifically because they suspect he is hiding there to avoid military conscription.

Luz is a woman living at the rebel camp who helps feed the group. She introduces Petra's family to her baby daughter and provides a brief sense of community amid the chaos of the war.

Chencha is the baby daughter of Luz. She travels with the rebel forces, representing the fragile civilian lives caught in the middle of the armed conflict.

Panchito is the family's donkey. He helps the girls transport the wood they gather to sell. Amelia secretly gives him a name and treats him as a close friend.

Don Raúl is a wealthy man in Petra's village. When his wife attempts to steal firewood without paying, he intervenes to apologize and compensates Petra directly.