Kara got a serious shock when she discovered the Daxamite Mon-El had crashed near National City at the beginning of the season.
That shock quickly turned to a reasonable amount of mistrust, which led to a grudging friendship, and then true friendship.
Kara forgave Mon-El for keeping his secrets after the musical crossover with The Flash taught her some lessons about love, and their relationship was going surprisingly well, even when his evil mom showed up to take him home.
Then, Kara and Mon-El started down a romantic path, even though (unbeknownst to Kara) Mon-El was still holding onto some major secrets which could have derailed their brand new relationship at any time.