In Matched by Ally Condie, who does Cassia end up with?

In Matched, Cassia doesn't end up with anyone; however, she does end the book intending to find and be with Ky if she can.
Cassia is happy to be matched with Xander at the beginning of the book. However, her focus and excitement wane when she sees Ky's information flash on her screen for a split second. That's the moment she starts to have doubts in the matching system and the society they live in.
At the end of Matched, Cassia decides to leave for the Outer Provinces. Ky has been sent there and people say that he might die in the war. She wants to find him and be in a relationship with him.
However, Matched is a trilogy. Cassia goes back and forth between her feelings for Xander and her feelings for Ky throughout the novels. At the end of the third book, Reached, Cassia chooses to be with Ky as her final choice of a partner.