Sister Aloysius is the principal of St. Nicholas School, a woman in her fifties or sixties who took her vows after being widowed in World War II. She operates with intense strictness and deep skepticism regarding human nature. She actively monitors the school for any signs of moral failure and instructs her younger teachers to maintain severe emotional distance from their students.

Father Flynn is a working-class priest in his late thirties from the Northeast. He teaches religion and basketball at St. Nicholas School, bringing a warm, progressive approach to his interactions with the boys. His sermons often feature parables to illustrate broader points about faith, and he actively pushes against the rigid traditions enforced by the school's principal.

Sister James is a young nun in her twenties who teaches eighth grade. Having recently transferred from an all-girls' school, she approaches her students with natural enthusiasm and idealism. She finds herself caught in the middle of a conflict between her desire to see the best in people and her principal's demands for constant vigilance.

Donald is an eighth-grade student and altar boy at St. Nicholas. As the first and only African-American student in the school, he faces distinct social isolation and physical danger. He relies on the positive attention of his parish priest to manage the difficulties of his daily life.

Mrs. Muller is Donald's mother, a highly practical woman in her late thirties. She views St. Nicholas School strictly as an academic stepping stone to high school and college for her son. She focuses entirely on the tangible benefits of Donald's education and avoids entertaining any discussions that might disrupt his progress.

Monsignor Benedict is the head of the rectory at St. Nicholas Church. The nuns view him as entirely oblivious to the daily realities of the school, representing the insulated male authority of the church hierarchy.

William is a young male student in Sister James's class. He goes home early in the school year with a nosebleed, an event the principal views with immediate suspicion as a potential behavioral tactic.

Sister Veronica is an elderly nun at the school whose eyesight is failing. She relies on the quiet protection of her fellow sisters to navigate the physical challenges of the campus.

Mr. McGinn is a staff member at St. Nicholas Church. He handles practical duties around the parish and is cited by the parish priest as a witness to an incident involving altar wine.