Catherine: A young woman that is 25 years old, who inherited much of her father's mathematical genius, and, she fears, his "instability" as well; she gave up her life and schooling to take care of her father until his recent death.
Claire: Catherine's older sister, a practical and business-minded woman who has been comfortably successful in her work and relationships. She left Robert and Catherine behind, distancing herself from the run-down family home of her youth. She left the family to make a new life for herself in New York City.
Robert: A recently deceased mathematician praised for his groundbreaking work in his youth, but whose later years were plagued by delusional mental illness; he is seen in Catherine's imagination and in flashbacks.
Harold (Hal) Dobbs: One of Robert's last Ph.D. students during the one year his idol and mentor's illness went into remission, at least enabling Robert to teach, if not continue his own creative mathematical work.