Spoilers ahead for Episode 19 of Supergirl Season 3, called "The Fanatical."
Supergirl has pulled out all the stops in its Season 3 supervillainess Reign. As a Worldkiller, she has handed down brutal beatdowns everybody ranging from the civilians of National City to Supergirl herself. The show has introduced a couple of other Worldkillers as well as alternate formidable foes, but Reign has undeniably been a big bad. After the events of "The Fanatical," however, we have to wonder if Supergirl is gearing up to reveal the Season 4 big bad sooner than we expected, and that big bad may be none other than Lena Luthor.
Although Lena has always been on the side of the good guys, the division between good and bad hasn't been quite as clear in the second half of Season 3. Her decision to keep Sam's identity as Reign a secret from Supergirl and the DEO was a decision made with the best of intentions, but Supergirl immediately judged Lena for keeping the secret. While both women had a point in their arguments, they didn't handle the conflict in the best way. Lena continued to keep dangerous secrets, believing that Supergirl is no longer to be trusted; by continuing to lie to Lena, Supergirl continued to dig herself deeper and deeper into a hole out of which even she may not be able to fly. Supergirl seems to be setting these women up for an epic confrontation.
In "The Fanatical," Kara was upset to discover that Lena was continuing to create technology that could be used against Kryptonians without disclosing that tech to Supergirl. Lena argued that she wasn't hiding any weapons from Supergirl, and she's not obligated to disclose all technology might be effective against Supergirl. The barrier she created to contain Reign zapped both Reign and Kara when they tried to use their powers against it. It was a good thing that Reign couldn't beat it at first, but Kara didn't see things that way. Since it could be used against her, she felt she deserved to know about it.
All of this said, Kara wanted to repair Supergirl's friendship with Lena, but Lena shot her down, saying that they were not friends and don't need to be friends in order to work together. Lena reminded Supergirl of her family history that means she doesn't get over betrayals of trust easily, and it's clear that Kara is beginning to understand just what will happen if and when Lena figures out that she is Supergirl. Lena is a very dangerous woman with a lot of resources; if she feels threatened by Kara as Supergirl, she could begin to consider her an enemy.
Now, many fans can probably see Lena's side in this whole conflict, and Supergirl deserves credit for showing why Lena is pulling away from Kara/Supergirl. That said, if these two characters do have a definitive split into enemies before the end of Season 3, we can probably safely say that Lena will be the villain. The show is called Supergirl, after all, and Lena's status as a Luthor has always made it likely that the show would explore some villainous roots. Given how quickly Lena and Supergirl's relationship is falling apart and the seeming inevitability that Lena will discover Kara's secret, we may discover that Lena will be the Season 4 big bad sooner rather than later. Supergirl is usually more ambiguous with introducing its big bads, but that may not be necessary if the new villain is a former friend.
We'll have to wait and see. There are still a few weeks of Supergirl left before a long hiatus, so be sure to tune in to The CW on Mondays at 8 p.m. ET to see what happens next. Kara and Co. will be in a brand new time slot when they do return to the small screen. Check out the changes to Supergirl and more Arrow-verse series on The CW's 2018-2019 fall schedule. For what you can watch in the meantime, take a look at our summer TV premiere guide.