They dated in season 2 but broke up. Kat realized that Christian still had feelings for Tara during their relationship. Kat decided to let him go. They should be together again, though, becasue Tara is now taken by Ben. She likes Ben. Ben likes her. It's a relationship, and Christian should accept it and move on by asking Kat out again (and only having eyes for her) even if he thinks he loves Tara. He also still loves Kat. But maybe he loves Tara more (which I personally despise). It's a love square: Kat, Christian, Tara, and Ben.
They have good chemistry and understand each other. Tara and Christian don't understand each other as well but do have history.
Kat liked Christian from the beginning.