Every time Kara falls in love with someone on Supergirl, fate finds a way to drive them apart. But even though she just found out that Mon-El is actually the Prince of Daxam, putting their relationship at the center of a centuries-old hatred between Daxamites and Kryptonians, maybe this romance could still work, even if Mon-El's royal family members just landed in National City. Besides, who are Mon-El's parents? Will these royals necessarily be against leaving the planetary discrimination aside and embracing their son's new girlfriend? Well, back in January, when Teri Hatcher was cast on Supergirl, TVLine reported that "she’ll eventually become the season’s big villain," though no other details about her role were known at the time.
Now that we know Hatcher is playing Mon-El's mother, Rhea, that could still be very true. The synopsis for the March 20 episode, "Star-Crossed," also calls Hatcher's character a "new villain," and the episode title evokes Romeo & Juliet, who were, of course, separated by their parents' still very active feud.
DC Comics can't provide too many clues, as Rhea is not the name of Mon-El's mother in any previous version of the character, and the only notable Rhea in its history is a metahuman who has electromagnetic powers. However, Mon-El's own comics history has changed throughout the years, so there's nothing stopping Supergirl from figuring out this problem and saving her new relationship. Mon-El's parents have never stood in his way before, but his own lack of information has. In this case, it's the lies he told Supergirl about who he is and how he got to Earth. Instead of a random guard on the run (or, in the comics, an amnesiac Daxan space traveler), he's a prince. If his mother is a villain, perhaps that explains why he was eager to leave his life behind.
Interestingly, Mon-El's father, Lar Gand, is not credited as a villain. He's named after Mon-El's real name in the comics, which identified him as a Daxamite when he was mistaken for a Kryptonian by Superman, and given the nickname Mon-El. It's possible that Lar Gand will either be less villainous and therefore more accepting of Kara, or, perhaps, he'll have the original Mon-El backstory, and be an explorer who is fascinated by other planets.
In newer comics, Mon-El was trapped in the Phantom Zone after being exposed to lead, which has a poisonous effect on Daxamites, so maybe either he or his father will be revealed as a former prisoner as well.
Whether Supergirl chooses to reveal that Mon-El was sent by his mother to destroy the Earth or he's on the run to get away from his destiny, his parents should be a total surprise. What's been revealed so far doesn't seem to match up with any previous comics history, so there's nothing to give away who Mon-El really is or what his parents motives are until Supergirl reveals them.