TV Line report that both Helen Slater and Dean Cain have been added to the cast of the CBS' Supergirl pilot in roles that shall remain a mystery. Both actors previously popped up in the CW's Smallville, where Slater played Clark/Kal-El’s Kryptonian mom Lara, and Cain appeared as the Dr. Curtis Knox). There's a good chance they'll both be playing established DC characters again, but for now we can only speculate about that.
So far the cast of Supergirl also includes Laura Benanti as Alura Zor-El, Chyler Leigh as Kara’s foster sister Alex, David Harewood as Hank Henshaw, Calista Flockhart as Cat Grant, Mehcad Brooks as Jimmy Olsen, and of course Melissa Benoist as the titular superheroine.