- Hazel Grace Lancaster: Hazel Grace Lancaster is a 16-year-old college student with lung cancer. She is depressed and knows that she'll die one day. She knows very well that being famous and loved widely does not matter as much as being who she is and being loved deeply because oblivion is inevitable.
- Augustus "Gus" Waters: Augustus Waters is a 17-year-old boy who suffered from cancer and subsequently had a leg amputation. Gus wants to be known widely and have people remember him when he dies. Prior to meeting Hazel, Gus had a girlfriend, Caroline Mather, who died from a cancerous brain tumor, and it is stated that Hazel resembles her.
- Isaac: Isaac is 17 years old and goes to the same support group as Hazel because of his eye cancer. His eye cancer has caused him to become blind.
- Peter Van Houten: Peter Van Houten is Hazel's favorite author, who wrote her favorite book, the fictional book An Imperial Affliction. Later he turns out to be an alcoholic who is neither planning to write a sequel for AIA nor willing to imagine a future for the characters in the book. He had an eight-year-old daughter who died of leukemia.
- Patrick: The strange support group leader who runs the meeting in the literal heart of Jesus at the church and had prostate cancer.
- Mr. and Mrs. Lancaster: They are Hazel's parents. Mrs. Lancaster's main job is to (as Hazel would say) "hover over her." She eventually plans to become a social worker and is already working for a year on her MSW. Mr. Lancaster is working for a real estate company – Morris Property Inc.
- Mr. and Mrs. Waters: They are Gus's parents. They have words of wisdom or Encouragements (as they used to call them) written all over their home. Gus's father along with Hazel agree that they have weird kids.
- Lidewij Vliegenthart: She is the assistant to the author Peter Van Houten who resigns in the latter part of the book. Lidewij is also a graduate student pursuing her PhD in American literature. She was the one who first wrote back to Augustus and made Peter reply to Hazel and Augustus's emails. It is Lidewij who takes them for a visit to Anne Frank's house and pays (on behalf of the author) for their dinner at Oranjee. After Augustus dies, she finds the last letters that Augustus wrote to Peter Van Houten before his death and emails them to Hazel.
Other characters include:
- Julie and Martha: Augustus's paternal half-sisters, both married to bankers called Dave and Chris. Among them, they have three boys.
- Graham: Isaac's ten-year-old brother.
- Kaitlyn: Hazel's only friend from her pre-cancer life. Apparently, she is the one who suggests that Gus might have written something and mailed somebody else.
- Caroline Mathers: Gus's ex-girlfriend who died of brain cancer before Gus and Hazel met.
- Drs. Maria and Simon: Hazel's doctors.
