Mon-El is a character that has been steadily evolving over the first half of the second season of Supergirl.
In the third episode, he was extremely hostile with an intense desire to return home to his planet of Daxam, a sister planet to Krypton.
Their prince sacrificed himself so Mon-El could escape the dying planet in a pod, which ended up crash landing on earth.
At first, the relationship between Kara and Mon-El was a bit like brother and sister, but as they spent more time together, a romance began blossoming.