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. The two planets had a long-standing rivalry and this caused initial conflict between Mon-El and Kara, who still retained that mutual prejudice for one another’s people. Mon-El revealed he was a palace guard for the Daxam Royal Family. Their prince sacrificed himself so Mon-El could escape the dying planet in a pod, which ended up crash landing on earth. Seeing an opportunity to fulfill the promise she never got to with Clark, Kara took Mon-El under her wing to teach him how to act like a human and learn to be a hero.
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. Though, it was more so from Mon-El’s end as we saw in the midseason finale when he was infected with a Kryptonian virus and in the midst of a half-conscious state, kissed Kara, before passing out again. Later on in the episode, after being cured, Mon-El seemed to infer that he didn’t remember the kiss, but the lingering camera shot on him before the scene’s end clearly suggested otherwise.
Television shows usually like to have an ongoing will-they-or-won’t-they drama, where one character possesses secret romantic feelings for another, while the other character isn’t in the same place as them. But finally once the other person does develop feelings and both attempt to move forward with a romance, an external problem is introduced to act as a roadblock.
Based on comments by Supergirl executive producer Andrew Kreisberg in an interview with EW, it sounds as though that external roadblock might be entering the picture which could stand in the way of Kara and Mon-El’s relationship. He was, of course, referring to Mon-El hinting at a secret about Daxam when he and Kara were trapped in cages at Cadmus in the episode, "The Darkest Place." Kreisberg said:
“Secrets are never healthy in a relationship and obviously Mon-El is holding onto something. We got a little taste that his life on Daxam might not be as over as we think from the glimpse of the space farers, who are on the hunt for him. So it will definitely play a big part in their relationship moving forward.”
It’s entirely possible that the story Mon-El told Kara about the day Daxam was destroyed didn’t play out exactly the way he portrayed it. Could it be that on the day Daxam was crumbling, Mon-El betrayed his prince and stole a pod to save his own life, leaving his former master to die? Is he being hunted for an act of treason? It wouldn’t be totally out of the realm of possibility given how Mon-El has handled himself on Earth, taking jobs like being an enforcer for a bookie, and his reluctance to be a hero. Could that reluctance stem from guilt he has about how he survived Daxam? Another theory suggests that Mon-El is actually the Daxamite prince. But don't expect a real answer from Kreisberg on this one since he told EW, "No comment."
Supergirl will return with "Supergirl Lives," a Kevin Smith-directed episode, on January 16, 2017.