Sister's Keeper is writen from the points of view of the main characters:
Kate - Kate Fitzgerald. Kate was born in 1988. At age 2, she was diagnosed with Acute Promyelocytic Leukemia. Leukemia is a cancer of the body's blood-forming tissue. We all remember from grade school that our white blood cells are our Warriors Against Disease. They keep us healthy. With leukemia, the white blood cells don't work. Kate is 15 in the story.
Anna - Andromeda Fitzgerald. Anna was born on the last day of 1990. Her parents had gotten pregnant with the specific purpose of finding a bone marrow match for Kate. They actually did genetic testing on the prospective embryos and selected the best match. They hoped that using Anna's umbilical cord blood would help Kate beat her leukemia. In the real life case on which Picoult based this story, the older child is still in total remission. Anna is 13 in the story.
Campbell - Campbell Alexander. Campbell is an attorney who seems to specialize in spectacular cases. He always has a service dog named Judge with him (although Campbell is not blind) and has a one-liner ready every time someone asks what the dog is for. His parents were wealthy but not particularly caring. He had a severe automobile accident at age 18. He is Anna's attorney.
Sara - Sara Fitzgerald is mother to Kate and Anna. She graduated from law school and was a civil attorney before her children were born, but left to be a full-time mother. She has an older sister named Suzanne who pretty much stood in as a parent and confidante after the death of their father and their mother's breakdown. She has spent the last 18 years being the best mother she can be.
Brian - Brian Fitzgerald is father to Kate and Anna. He is a fire-fighter and captain at his station. He loves astronomy and named Anna after the Andromeda galaxy.
Jesse - Jesse Fitzgerald is Kate and Anna's older brother. He's gotten lost in the family. Most of their attention is focused on Kate and Anna. He's 17 years old, and is quite taken with fire. He loves his siblings a lot, and really, really wants attention.
Julia - Julia Romano is Anna's guardian ad litem (GAL). A GAL is a child's advocate in a legal proceding involving a child. Julia is an attorney (although she doesn't need to be one to be a GAL). She had previously been doing non-profit work, but her income wasn't covering her bills. So Julia became a GAL. She has a twin sister named Isobel who has just moved in with Julia after Isobel's relationship collapsed.
Some of the other characters (who don't speak directly to us) are:
Judge DeSalvo is presiding over the case. His daughter was killed by a drunk driver the previous year.
Kerri Donatello is Campbell's secretary.
Judge is Campbell's service dog.
Dr. Chance is Kate's oncologist.
Isobel (Izzy) is Julia's twin sister.
Suzanne (Zanne) is Sara's older sister.
Taylor Ambrose is another cancer patient that Kate knows. They go to "hospital prom" together.
All of the main action in the book takes place in an eleven day period - from Monday of one week through Thursday of the following week. That's sometimes hard to remember because the book flashes back to 1990 and forward to 2010. But the main action all happens in eleven days.