Oscar is a 10-year-old cancer patient who lives at the hospital. Nicknamed "Egghead" for his baldness due to treatments, he possesses a self-deprecating and highly realistic perspective on his illness. Feeling alienated by the fearful avoidance of the adults around him following a failed bone marrow transplant, he directs his sharp, honest questions to God. He copes with his environment by observing the hospital's daily life and writing letters that treat each of his remaining days as an entire decade of life.

Granny Rose is an elderly volunteer at the children's hospital, recognizable by her pink uniform. Unlike the medical staff and Oscar's parents, she does not avoid the topic of illness and treats Oscar with straightforward honesty. She entertains him with wild, anecdotal stories from her supposed past as a professional wrestler. Her unconventional approach provides Oscar with a crucial outlet for his fears and a structure for processing his remaining time.

Peggy Sue, widely known as Peggy Blue on the ward, is a young female patient at the hospital suffering from a blood disease that gives her skin a bluish tint. She is disturbed by cries she hears at night and seeks comfort in the hospital environment. She becomes the focus of Oscar's affections, accepting his offer to protect her from ghosts, and assumes the role of his primary romantic interest as he mentally lives out the decades of his life.

Dr. Dusseldorf is the lead physician treating Oscar at the hospital. He struggles emotionally with his inability to cure his young patient following a failed bone marrow transplant. His feelings of guilt and disappointment over the failed treatments cause him to avoid eye contact with Oscar, inadvertently contributing to the boy's sense of alienation.

Oscar's mother lives far from the hospital and typically only visits on Sundays. Overwhelmed by the reality of her son's failing health, she struggles to communicate with him and often avoids direct conversations about his condition. Her avoidance stems from profound fear and sadness rather than a lack of love, though Oscar initially interprets her actions as cowardice.

Oscar's father struggles with his son's terminal cancer diagnosis, manifesting his grief through physical and emotional distance. Because he doesn't know how to act around his sick child without showing his intense sadness, he frequently stays away from Oscar's hospital room. His inability to confront the situation directly creates a tense rift between him and Oscar early in the story.

Popcorn is a fellow pediatric patient at the hospital. He is part of Oscar's social circle on the ward and initially stakes a claim on Peggy Sue, acting as a minor obstacle to Oscar's budding romance. Like the other children, he accepts his descriptive nickname, given due to his weight, without complaint.

Cleopatra is a young girl receiving treatment in the hospital. She briefly becomes involved in Oscar's romantic life when he kisses her out of frustration over Peggy Sue. This action causes social friction on the ward and temporarily complicates Oscar's relationship goals.

Bacon is a patient suffering from severe burns that cover his body. He is the true source of the frightened crying that Peggy Sue hears at night, which the other children initially mistake for ghosts. He is part of the community of sick children who refer to each other by symptom-based nicknames.

Einstein is a fellow pediatric patient recognizable by his unusually large head. His nickname serves as a symptom-based joke rather than a description of his actual intelligence. He forms part of the background community of children on the hospital floor who face their physical conditions with casual acceptance.