Calista Flockhart was a regular for all of the season one of Supergirl. But with the shows move to the CW and filming in Vancouver, the character of Cat Grant suddenly walked away from her company after two episodes, leaving James Olsen (Mehcad Brooks) in charge. The producers have said that a return has always been in the cards, but now TVLine is reporting that return will take place in the last two episodes of the current season.
This continues the parade of returning characters being announced for the Arrowverse's finales. Supergirl is also getting an appearance from Tyler Hoechlin as Superman and Arrow is getting the return of Katrina Law as Nyssa al Ghul, Manu Bennett as Slade Wilson, and possibly Nick E Tarabay as Digger Harkness (unconfirmed). No returns have been announced for The Flash… yet.