Warning: spoilers ahead for the latest episode of Supergirl, entitled "Crossfire."
Supergirl raised the stakes in Season 2 when the nefarious organization known as Cadmus began interfering with matters in National City. A mysterious doctor has been in charge of the misdeeds at Cadmus, including the creation of the villain Metallo that nearly took down Supergirl and Superman. In "Crossfire," the doctor got her hands even dirtier by distributing alien weapons to human bad guys to wreak havoc and set the public against the Alien Amnesty Act. The real surprise came when the final moments of the episode revealed that the dastardly doctor and apparent Season 2 supervillain is none other than the mother of Lena (and presumably Lex), Lillian Luthor.
One of the biggest, baddest, and scariest villains of DC Comics is Lex Luthor; even folks who have never picked up a Superman comic book know him as the Man of Steel's greatest enemy. When news broke over the summer that Supergirl Season 2 would be involving Lex's sister Lena, it seemed likely that the show was either building to the introduction of Lex as a Supergirl villain or using Lena as a Lex surrogate. After all, the show added Superman into Season 2 when he had previously been ruled out; who was the say that the show wouldn't bring in Lex in the wake of his lesser-known little sister? But no, the show can still have Lena's cake while icing it with a deepening bloodline.
The twist that Supergirl is using the Luthor matriarch (played by Brenda Strong) rather than Lex or Lena as a big bad could be pretty fantastic for a couple of reasons. The first is just that DC Comics has done a fair amount with Lex and Lena on the page; their mother has scarcely been mentioned. There is no comic canon to guide or constrain the producers moving forward, so the character can be as shifty and sinister as the Supergirl team can imagine. The closest that DC Comics comes to connecting Lillian Luthor to Cadmus is when Lex Luthor's ex-wife ran a division of it.
Another reason why El Madre de Luthor could be a fabulous villain is that the show may be crafting yet another nuanced female baddie. Supergirl excelled in the first season at writing female villains who were more than tight costumes, long hair, and quips. Now we know that Lena's mother is the doctor behind Cadmus' crimes, she has the potential to become one of the most unforgettable characters of any of The CW's DC shows. She already has a motivation beyond simply wanting to see the world burn. She wants to save the world for her son (presumably Lex) and her daughter, and she believes that sending all aliens away from Earth is the way to do it.