Katy is an intelligent, empathetic eight-year-old growing up in a prosperous household in 1910. As an only child who frequently accompanies her physician father on his patient visits, she learns early about the social and economic disparities in her rural community. She resolves to become a doctor herself and possesses a sharp ability to quietly observe the behavior of the adults and children around her.

Jacob is a thirteen-year-old boy who does not speak, communicating instead through rocking movements and precise imitations of animal and machinery sounds. Kept out of school and dismissed by many townspeople as an imbecile, he focuses his time and gentle energy on caring for farm animals. He finds comfort in routine, wearing a distinct cap, and observing the rhythmic noises at Schuyler's Mill.

Dr. Thatcher is the town's benevolent and dedicated physician. He treats his young daughter with immense respect, explaining medical realities to her honestly rather than relying on euphemisms. He approaches marginalized people in his community with the same open-minded compassion he models for his family.

Mrs. Thatcher is a patient, loving maternal figure who maintains a warm, traditional home. She ensures Katy feels secure and included as the family anticipates the birth of a new sibling. While she conforms more strictly to the social norms of the early 1900s than her husband, she provides a foundation of steadfast kindness.

Peggy is a pragmatic teenage farm girl hired to assist the Thatchers' cook, Naomi. Forced by economic necessity to leave school at age fifteen, she embraces her domestic duties and acts as a comforting second mother figure to Katy. She harbors quiet aspirations of saving money and eventually establishing a household of her own.

Nell is Peggy's sixteen-year-old sister, marked by her brash defiance of her working-class farm origins. She despises her domestic position and dreams of upward social mobility, imagining herself as a future movie star under the stage name Evangeline Emerson. She actively pursues the attention of the older boys in the neighborhood, heedless of social boundaries.

Paul is the teenage son of the affluent Bishop family, living next door to the Thatchers. He resents his parents' strict expectations that he attend college and become a lawyer. He demonstrates his rebellion by flirting intimately with the family's hired girl in the barn, pushing the boundaries of his community's rigid class rules.

Austin is the younger son of the Bishop family and Katy's classmate. Outside of school hours, where boys and girls are kept separate, he frequently plays elaborate imagination games like "Tragedy and Disaster" with Katy in their adjacent yards.

Mr. Bishop is a successful, educated lawyer who enjoys machinery and modern inventions. He is the first man in the neighborhood to purchase an automobile, reflecting his family's wealth and status, though he maintains rigid, traditional expectations for his older son's future.

Jessie is Katy's classmate and frequent playmate. Unlike the composed Katy, Jessie tends to be loud, easily frustrated, and impatient. She holds narrow views of proper behavior and openly questions Katy's efforts to befriend individuals who are different from them.

Mr. Stoltz is a stern, hard-working farmer who demands obedience from his children. He struggles to understand Jacob's sensitivities and expresses frustration when his son runs and hides during practical farm tasks like butchering.