Hazel Grace Lancaster
Kept alive by an experimental drug, Hazel is unsure how long she has left to live and struggles to be defined by more than her fight against cancer and her survival. She finds in Gus someone she can relate to and who helps her live outside of herself. The trials and revelations of a girl fighting cancer in a book called An Imperial Affliction obsesses her until she realizes the author is not the person she imagined. She then decides to live the rest of her life not like the girl in the book but as herself, living her own life even more fully.
Augustus "Gus" Waters
Gus falls in love with Hazel and decides to use his Wish to take her to Amsterdam even as he knows he is dying from a recurrence of cancer spreading throughout his entire body. He is afraid he will die without being known for anything special but comes to realize all he needs is Hazel's love and her recognition of his worth.
Isaac initially introduces Hazel to Gus at a cancer support group meeting just as he learns he will become blind in two weeks. His girlfriend breaks up with him prior to his surgery, and he remains angry for a large portion of the book because of it. Unlike Gus and Hazel, he shows his emotions, whether they are his love for Monica or his hurt and anger at her betrayal, and publically and loudly, in sharp contrast to the widespread idea that cancer patients are silent fighters, courageous throughout their treatment.
Mrs. Lancaster is Hazel's sometimes overly close mother and caretaker, knowledgeable in all of the necessary cancer-related information. She encourages Hazel to interact with the outside world whether it is with the cancer support group, Kaitlyn, or Gus. At the same time, she is going to school to become a counselor to help other families afflicted by cancer. However, she doesn't tell Hazel initially because she doesn't want her to feel abandoned or rejected in the rest of her life and time.
Peter Van Houten
Van Houten initially wrote An Imperial Affliction to give his daughter the teenage years she would never have because of leukemia. While he initially inspires Hazel because of his seeming insight into living with the struggle against cancer, he quickly becomes the realization of her worst fears. The death of his daughter turned Van Houten into a miserable alcoholic, and Hazel fears what will happen to her own family after her death.