Anna stands out as the book’s most conflicted character. Her connection with Kate, and her struggle to exist independently of that connection, both define her. She tells Campbell, for instance, that of all the things she might want to be in ten years, what she most wants to be is Kate’s sister. At the same time, Anna desperately wants to exist independently of Kate, but she knows she cannot do so as long as her main purpose in life consists of keeping Kate alive. These contradictory feelings make up the tragic core of Anna’s character. She feels a tremendous sense of guilt for wanting to live separate from Kate and wonders if she is an awful person for feeling that way. As if trying to roleplay the part of an awful person, she even begins to indulge in self-destructive behaviors, such as smoking, with Jesse. But Anna also wants to do what’s best for her sister. Kate, we learn, ultimately decides that she no longer wants to live, so Anna, although it hurts her deeply, brings a lawsuit against her parents for medical emancipation. The lawsuit satisfies both desires: it gives Anna control of her own body, allowing her to put her own interests before Kate’s; and since Kate will die without Anna’s kidney, Anna can fulfill Kate’s wish to die.
Anna also represents the point where science and humanity intersect. Her parents conceived her—with the aid of scientists—for a very specific reason: to provide Kate with a genetic match whose organs could help keep Kate alive. Sara even admits she could only think of the unborn Anna in terms of what she could do for Kate. Despite this scientific reason for Anna’s existence, she clearly amounts to more than just a donor, both to her family and to the reader. Anna is funny and thoughtful, described by Brian as the family’s constant and source of light. She has contributed to the Fitzgerald family on far more than a medical level. Her emotional attributes have helped Kate just as often as her physical ones. Thus, Anna’s life suggests that no matter how far science advances in its ability to engineer humans for a purpose, those humans are still thinking, feeling people who will always mean more than just their scientific reason for being.