Jesse Rath has been upped to a series regular for Supergirl season 4. That means at least one member of the Legion of Super Heroes will presumably be staying in the 21st Century, and presumably working alongside Supergirl and the DEO.
The fan-favorite Legion of Super Heroes were first teased back in Supergirl season 1, when attentive fans noticed a Legion Flight Ring in the Fortress of Solitude. The Legion finally made their appearance in season 3, with a handful of Legionnaires siding with Supergirl in the battle against the Worldkillers. Although the Legion headed back to the future, the trailer for the next episode has promised they're about to return to help fight against Reign.
TVLine has revealed that Jesse Rath, who plays Brainiac-5, will become a series regular in season 4. "We love the character of Brainiac-5 and can’t imagine anyone but Jesse Rath in the role," executive producers Robert Rovner and Jessica Queller observed. "He brings such fun, heart and brilliance to Brainy, and we’re so excited he’s joining the cast as a series regular for season 4. We have a lot of great Brainy stories to tell." Viewers were initially critical of Supergirl's version of Brainiac-5, largely because of his skin color (which isn't comic-book-accurate), but Rath's portrayal has won fans over.
Supergirl is typically focused around Kryptonian science, but Brainy's science is beyond even that of Kara's homeworld. He prides himself on being a 12-level intellect, and is the inventor of much of the Legion's advanced technology. Adding him into the mix should give season 4 a unique dynamic, and allow the show to move beyond Kara's Kryptonian heritage. Of course, the interesting question is whether or not other members of the Legion will become series regulars as well - including Saturn Girl, Mon-El's wife. It's also possible we'll finally meet other members of the Legion, perhaps including Chameleon (who's already been referenced) and Cosmic Boy.
Interestingly enough, Brainiac-5 could actually prove to be an important character in other ways. Supergirl's Legion is inspired by Kara's example, and Brainy has already demonstrated his fondness for the Girl of Steel. In the comics, though, the two go further; Brainy is actually one of Supergirl's most important recurring love interests. A hidden love of Kara could neatly explain why Brainiac-5 either stays in the present at the end of season 3, or else travels back in time again next season. Next week's season finale will no doubt set the stage for season 4, so we won't have long to wait till we learn more.