Kim is Mia's best friend. Having too much in common initially made them hate each other, but they eventually realized they were perfect for one another.
Kim must be a Libra, because she likes to keep things in balance. Mia is crazy about Adam. Kim is lukewarm about him. Kim's mother is hysterical at the hospital, but Kim keeps her cool. Kim "makes light of her religion" (7.20), but at the same time, she takes her Jewishness seriously and prays for Mia's recovery.
We only see Kim in relation to Mia, because the whole world revolves around Mia. In that respect, Kim is the perfect friend. "Kim had to bear" (10.6) the pain of her friend being injured, but she does it calmly. And she helps by retrieving Adam from his concert to bring him to Mia's side.
Unlike Mia, who will drop Kim and Adam in favor of her music career, Kim seems to put her friends first. Who makes the right decision? That's for you to decide.