Hazel is a 16-year-old girl living with thyroid cancer that has spread to her lungs. Her medical condition requires her to carry an oxygen tank everywhere and limits her physical endurance. Highly intelligent and naturally cynical, she spends much of her time reading, particularly her favorite novel *An Imperial Affliction*. She actively resists forming close relationships outside her family, viewing herself as a "grenade" whose terminal illness will inevitably cause pain to those she loves.

Augustus is a 17-year-old former basketball star who lost his lower leg to osteosarcoma. Currently showing no evidence of cancer, he possesses a native optimism and a humorous, carefree outlook on life. He is deeply preoccupied with making a heroic mark on the world and fears oblivion above all else. He actively pursues a connection with Hazel despite her hesitations, undeterred by the potential future pain he might cause himself or others.

Peter Van Houten is the American author of Hazel's favorite book, *An Imperial Affliction*. After publishing his only novel, he moved to Amsterdam and has not published anything in a decade. He serves as an intellectual idol for Hazel, who is desperate to ask him about the unresolved fates of the characters in his book. His correspondence sets a major trip into motion.

Isaac is Augustus's best friend and a fellow cancer patient who faces the imminent loss of his remaining eye to the disease. Like Hazel, he uses sarcasm to cope with the difficulties of his condition. His experiences highlight the compounded struggles of dealing with normal teenage angst alongside a severe medical diagnosis, particularly regarding his romantic relationships.

Hazel's mother is deeply devoted to caring for her only child, often practicing "celebration maximization" by inventing holidays and observing every possible milestone. She constantly monitors Hazel's health and attempts to facilitate normal teenage experiences for her daughter. Her entire routine revolves around Hazel's complex medical needs.

Hazel's father is a biochemist who is intensely emotional and deeply affected by his daughter's illness. He holds philosophical views about the universe wanting to be noticed and provides Hazel with a comforting, observant presence in her restricted routine. He is prone to crying over Hazel's medical condition.

Patrick is a cancer survivor who leads the support group for teens with cancer in a space he affectionately calls the "Literal Heart of Jesus." He frequently shares his own experiences with the disease and relies on conventional, sentimental rhetoric about fighting, winning, and survivorship that Hazel often finds grating.

Kaitlyn is a sophisticated and popular girl from Hazel's past, before Hazel became too sick to attend regular high school. She represents the "normal" teenage world of boyfriends and school drama. Hazel often feels awkward and alienated during their interactions, acutely aware of the social divide created by her illness.

Lidewij is the Dutch assistant to the reclusive author Peter Van Houten. She manages his correspondence and actively helps arrange the logistics for Hazel and Augustus to visit Amsterdam, acting as a bridge between the erratic author and his young fans.

Monica is Isaac's girlfriend at the beginning of the novel. Their relationship provides a lens into how healthy teenagers handle the heavy reality of their partners' illnesses. She and Isaac frequently share the phrase "always" as a promise to each other.

Caroline is Augustus's former girlfriend who passed away from brain cancer. Her memory and the way her illness affected her personality serve as a cautionary tale for Hazel about the emotional pain of loving someone with a terminal disease.

Augustus's mother is a welcoming figure who decorates her home with "Encouragements," positive inspirational quotes crocheted and printed on various objects. She is deeply protective and supportive of her son, preferring to focus on hope and positivity.

Augustus's father is a warm man who appreciates Hazel's ability to bring out his son's sense of humor. He faces the difficult reality of his son's medical history by leaning heavily on his family and maintaining a cheerful household.