The Narrator is a twelve-year-old boy living in a working-class neighborhood on the outskirts of Nogales, Arizona. He finds the academic and social expectations of junior high school overwhelming, especially without the guidance of a single primary teacher. To cope with his frustrations, he frequently visits a dry arroyo to yell into the open air. He feels a strong attachment to perfect, round objects, which represent the childhood simplicity he desperately wants to maintain.

Sergio is the Narrator's next-door neighbor and closest confidant in Nogales. He participates in all of the Narrator's schemes, from building branch forts in the creek bed to packing unnecessary silverware for a three-day hiking trip. He shares the Narrator's working-class background and his intense curiosity about the world beyond their immediate neighborhood. Sergio operates on the same intuitive wavelength as his friend, understanding complex feelings without needing to put them into words.

The Mother is a practical, firm parent raising her son on the Arizona-Mexico border. She recognizes the dangers of their local environment, actively warning the boys away from the polluted creek and the distant hills. She speaks in fast, compressed sentences that the boys often struggle to interpret. While she lays down strict boundaries, she also recognizes when she cannot stop the boys from learning hard lessons on their own, eventually allowing them to pack their rucksacks and walk into the hills unhindered.