Cat left National City back in the second episode of Season 2 because she felt she had become the biggest fish in a small pond and needed to seek challenges elsewhere; it’s a safe guess that either she’s returning because of a huge splash in National City that brings new challenges or because she wants to check back in on CatCo.
Calista Flockhart is slated to appear in the May 15 penultimate Season 2 episode and the May 22 season finale, according to EW.
Unsurprisingly, nobody at The CW or Supergirl seems willing to dish any details about what Cat will be up to when she drops back into the action, but we can bet that she’ll be back for a good reason.