Vivian is a fifty-year-old professor of seventeenth-century poetry specializing in the metaphysical works of John Donne. Recently diagnosed with advanced ovarian cancer, she agrees to an aggressive experimental chemotherapy treatment overseen by university doctors. She prides herself on her fierce intellect and rigorous academic standards, which have brought her professional success but left her without a personal support network.

Jason is a twenty-eight-year-old clinical fellow in the medical oncology branch at the university hospital. He previously took Vivian's seventeenth-century poetry course to prove he could ace the hardest class on campus. Highly driven by the pursuit of medical knowledge, he views cancer as a fascinating scientific puzzle and plans to transition entirely into pure research once his mandatory fellowship ends.

Susie is a twenty-eight-year-old registered nurse who serves as Vivian's primary caregiver throughout her chemotherapy regimen. She operates in sharp contrast to the research-focused doctors on the ward. She actively seeks to make her patients comfortable and checks in on their emotional state during difficult treatments.

Dr. Ashford is an eighty-year-old professor emeritus of English and a renowned scholar of John Donne. Decades earlier, she served as Vivian's mentor and shaped her rigorous approach to literary analysis. She demands absolute precision from her students but also possesses a maternal tenderness, occasionally encouraging them to step away from the library to enjoy life.

Dr. Kelekian is the fifty-year-old Chief of Oncology at the university hospital. He prescribes an aggressive, eight-month experimental chemotherapy protocol for Vivian. He leads the Grand Rounds, instructing medical students on the clinical specifics of the ward's patients while maintaining a detached, purely scientific approach to healthcare.

Mr. Bearing is Vivian's father, seen exclusively in a brief memory from her childhood. He encourages her to sound out unknown words and understand their meanings, inadvertently sparking her lifelong obsession with vocabulary and language.