The characters of My Sister's Keeper feel like they could be divided into two categories: those who hurt (Jesse, Sara), and those who help. Julia is one of the helpers, saying point-blank, "helping someone who needs it is the right thing to do" (3.6.78). Of course, things aren't that black and white, and even Jesse and Sara are trying to help people occasionally. Plus, if we're going to be totally honest, Julia isn't actually that much help at all.
She's a very nice lady, don't get us wrong, but her role as guardian ad litem to Anna is almost completely superfluous. (Spoiler: Julia isn't even in the movie.) She takes Anna to the zoo, and she talks to Campbell about the cast a lot, but she never really does anything. By the end of the case, she doesn't even make a decision about what's best for Anna.
Julia's main role in the story is as Campbell's love interest. See, they were pretty hot and heavy in college until Campbell ditched her and Julia spent a couple of decades thinking she was a useless human being, grousing to Seven, the one heterosexual bartender at a gay bar about how she's a "train wreck of sexual history" (4.2.135), and to her twin sister, Izzy, who seems to only exist to argue with her sister about relationships.
But Campbell was her first love, and she will always love him, like Whitney Houston. Campbell used to call her Jewel, maybe because she will save his soul, or her hands are small, but mainly because it sounds like a shortened version of Julia. (Sound it out, Shmoopers.)
While Julia is supposed to be doing whatever a guardian ad litem does, she ends up sleeping with Campbell again on his sailboat and getting ditched on the high seas. Eventually, she realizes that he's dumped her because he has epilepsy, and she tells him what we're all thinking: That is the dumbest reason ever to dump someone. In the epilogue, we find out that they're married, but not what Julia is doing with her life. Probably spending all her time remodeling Campbell's terrible apartment…