Supergirl's fifth season may be a little bit shorter than originally intended thanks to the coronavirus pandemic, but there's still a lot for fans to look forward to. Not only is it set to see Supergirl (Melissa Benoist) reach out to estranged friend Lena Luthor (Katie McGrath) to stop Lex Luthor (Jon Cryer), the episode will also see the return of the female Brainiac-5 played by series star Jesse Rath's sister, Meaghan. Rath told ComicBook.com that there's a "lot of cool Brainy stuff" coming in the upcoming "Immortal Kombat" and that he was very happy that includes his sister's return to the show.
"I'm happy with that because I think [episode 619] is such a strong episode," Rath said. "It's a lot of cool Brainy stuff. You see Brainy's plan come to fruition in that episode. And my sister, Meaghan Rath, returns to the show in  so I'm happy that  will be the finale. I think it's a strong episode and it'll do it justice."
He went on to say that working with his sister to play two versions of the same character is something that he had enjoyed greatly, especially when they had to work out earlier in the season how Meaghan would approach her female version of Brainy.
"It's been a lot of fun. I always love working with Meaghan. I feel honored that she came and did this as a favor to me," Rath said. "She's very busy, especially at the time we were shooting. It was crazy hectic, the schedule, so the fact that she came down and we were able to make it work, it means the world to me. It's great. It's funny that we're kind of playing the same character. When we were figuring out what her character was going to be and how she was going to play it, there was a lot of... she toyed with the idea of maybe doing a direct impression of me as Brainy and I think in Episode 10 we realized that the way Brainy acts, his mannerisms, his voice and everything, it's almost a product of his inhibitors that was put on him.
"The fact that the other Brainy didn't have these personality inhibitors meant that they weren't going to necessarily act the same as that Brainy," he continued. "It ended up being nice... we ended up deciding that it was best for her not to try to do a direct impression of me and just more so the essence of the character. And I think it's awesome. I love it whenever I get to work with her, but this is especially special. I think it's awesome. I had a blast and I can't wait for you all to see."
Meaghan Rath made her Supergirl debut in "The Bottle Episode" in which a number of doppelgangers from Earths destroyed during "Crisis on Infinites Earths" began showing up in National City. Before deciding to join "the Big Brain", her version of Brainy gave Rath's some life-altering advice about how to deal with Leviathan: to save his world, he'd have to work with Lex Luthor. Since then, Brainy has been doing exactly that, but it's something that's been taking a huge toll.
"I think he's not doing too well," Rath said. "I think it weighs on him a lot, the fact that he feels maybe like he's betraying his family and friends. It's like he almost has to work against his own self-interest. Everything tells him that he shouldn't be trusting Lex, but it's almost like he has to do the opposite of everything he thinks is right. He wants to lean into that friendship or alliance with Lex in order for things to work out in the end."
Supergirl airs Sundays at 9/8c on The CW.