Kate is The Sick One. She has "acute promyelocytic leukemia" (1.1.18) and is dying from the moment we meet her until almost the very end of the book. She's sick and tired of being sick and tired, though, because she has almost always been The Sick One and Sara watches over her like she's inside a giant invisible bubble. All the attention can be stifling.
One moment stands out in Kate's memory, and that's the time she accidentally got tackled in a game of football with her family. Jesse forgets that he's supposed to handle Kate with latex gloves and tweezers and sacks her butt into next Thursday. Of course, Sara gets mad, but Kate is fine. She's better than fine, in fact—she says, "It doesn't hurt. It feels great. […] He forgot" (3.7.80, 3.7.81). And when Jesse forgets that Kate is sick, Kate, too, gets to forget that she's sick for just a second, which is almost as good as not being ill.
Another highlight in Kate's life is her brief boyfriend, Taylor Ambrose. He's also sick, so he understands how difficult it is to go through chemotherapy and readily does what few other guys would do: "holds the vomit basin beneath [Kate's] chin" (7.3.74). Most boys that age can't even remember to pick up the phone and call their girlfriends.
He's Kate's first kiss, and he tastes like "Popcorn […] and guy" (7.3.74). He asks her to a hospital dance for all the sick kids, and she gets to pretend to be a "normal" girl going to the prom for once. Yay.
But don't get too comfortable with the good times, Shmoopers, because then he dies.
It's pretty sad, if not unexpected, that he would die. What is unexpected (or not, depending on how you feel about her) is that Kate's mother, Sara, keeps it from Kate, making Kate think that nice Taylor is just a jerk who is standing her up after the prom instead of, you know, not calling because he's six feet underground. This shows us how Sara sometimes goes too far to protect Kate, and just ends up hurting her more. Sigh.
Perhaps the most shocking thing about My Sister's Keeper is that Kate ends up living and Anna, the donor, dies. Not during organ donation, either, but in a freak car accident. After this, Kate receives Anna's kidney and ends up living.
Kate kind of feels guilty about this. If you're a believer in fate, it might seem like Kate altered it—it was her idea for Anna to sue, because Kate wanted to die. So if she hadn't done that, she wouldn't have been in that car, and, as Kate says, "[Anna] would be here, and I would be the one coming back to haunt her" (Epilogue.3). Perhaps.
It's a nifty little role reversal when you think about it. Kate becomes her sister's keeper in a way—the keeper of Anna's photographs, her memories, and, well, her kidney.