- Andromeda (Anna) Fitzgerald: Was conceived by IVF to be a genetic match for her sister, Kate, who was diagnosed with acute promyelocytic leukemia when she was two years old. Throughout her life, she donated her sister cord blood, lymphocytes, granulocytes, and bone marrow to help save her sister. When she is 13, she is asked to donate a kidney which she refuses to do and seeks the help of Attorney Alexander Campbell to medically emancipate her from her parents. Throughout the story, she is depicted an incredibly intelligent and witty young girl who like all children that age, is stuggling to find out who she is. She has an incredible talent for hockey however she has to put a lot of it on hold to be there for her sister. She receives her medical emancipation at the end of the story, however the very day she receives it, she is involved in a fatal car accident which leaves her brain dead. Her kidneys are donated to her sister, however her death shakes up the family and leaves them in shambles for many years afterwards.
- Katherine (Kate) Fitzgerald: Was diagnosed with acute promyelocytic leukemia when she was only 2 years old. Since then, she has undergone numerous treatments in an attempt to prolong her life, including chemo, ATRA (ALL-TRANS Retinoic Acid), and arsenic therapy. She has also received cord blood, lymphocytes, granulocytes, and bone marrow from her younger sister, Anna. When her kidney's begin to fail, her parents talk about having Anna donate a kidney however Kate secretly tells Anna not to do it. She explains how she has spent a good portion of her life in and out of hospital and that she can't stand to do it any longer. She never tells Anna to instigate a lawsuit but she does tell Anna to stop being her donor and to let her die. However, when Anna gets into a car crash and becomes brain dead, Anna attorney gives the doctor''s permission to donate her organs, giving Kate the kidney she needs. In the epilogue, she says she has been in remission for 8 years, thanks to her sister's kidney and now teaches at a ballet school. She believes that the reason she did not die was because Anna took her place in Heaven. In the story, it is mentioned numerous times how much she looks like her father, Brian, and her love for soap operas and reading.
- Jesse Fitzgerald: Jesse is the older brother of Kate and Anna. In the story, as Kate becomes sicker and sicker he feels more and more left out and abandoned by his parents, who admit that they had completely given up on Jesse long ago. Struggling to cope with the idea of losing his sister and trying to gain his parents attention, he lashes out by ripping his braces off with a fork, wandering into the city and standing underneath the traffic light of an intersection, stealing cars, and more recently by drinking and doing drugs. He has also begun to light old abandoned buildings on fire just for fun however, when he sets part of an elementary school on fire, his father, one of the firefighters on the scene, recognizes on the cigarettes left on the scene as ones that Jesse smokes. When he confronts Jesse, he eventually breaks down and cries in Brian's arms. After that, he spends more and more time at the hospital watching over Kate, especially when his parents are in court. After Anna's death, Jesse changes his ways and becomes a police officer. Throughout the story, it is mentioned how much he mirrors his mother, Sara, in appearance however deep down he feels most closely related to Anna, as he feels they are both the same deep down.
- Brian Fitzgerald: The father of Jesse, Kate, and Anna and the husband of Sara. Brian first met Sara during a flood outside one of the courthouses where Sara worked as a lawyer. After bringing Sara to safety, they fell in love and got married. When Kate became diagnosed with leukemia, he is obviously distraught and like his wife, continues to search everyday for a new cure that could save her. As the years of Kate's illness progress she spends more and more time at the firehouse as fighting fires not only gives him something to do to keep his mind off Kate, but because it is a situation in which he feels more in control of. He is a closet astronomer whose car is filled with star charts, pictures of nebulas, and other such things. It is he who names Anna, constellation, Andromeda, which according to legend is placed between her mother and father. He loves all of his kids equally, however he supports Anna decision for medical emancipation and allows her to move into the fire house with him during the trial. However, once he gets on stand he admits that he wishes Anna would donate a kidney however, he says that he knows that decision would be the right one. It is Brian who gets a call about a car wreck the night Anna receives medical emancipation. He goes the scene only to see that the people involved in the car wreck were none other than Anna and her attorney. He removes her from the wreck and rides with her in the ambulance only to see her flat line. He manages to revive her however he is informed after getting her to the hospital that Anna is brain dead. After her death, it is mentioned that he "fell into the bottle" for a while and even though he has recovered now, he hasn't been the same since.
- Sara Fitzgerald: Sara is the mother of Jesse, Kate, and Anna and the wife of Brian. As a child, her father died of a heart attack while mowing the lawn and afterwards, her mother became to depressed to function. This left her primarily in the care of her older sister, Suzanne (Zanne) whom she looked up to throughout her childhood. It is Zanne who helped her keep her grades up and who encouraged her to go to law school and dream big. When Sara married Brian (who had not gone to college or done anything professional) and gave up her law career to have children, Zanne was somewhat angry and they stopped talking for a time. When Kate became diagnosed with cancer, Sara called her sister for support and their relationship was healed. Throughout the story, Sara has been the biggest supporter of Kate and continually stands by her side and seeks out any possible treatment. Even though she is not aware that she is doing so, she has begun ignoring Jesse and Anna when it comes to Kate and her health condition.