Just got wind that Chris Vance was cast as the Kryptonian Ex-Scientist Non. The description they have of him is pretty scary and means trouble for Kara:
... a former scientist in a league with the House of El. Non is a brutal Kryptonian military officer who is sinister, powerful and angry. Non is said to represent the opposite of everything Supergirl (Melissa Benoist) stands for, and will become her greatest threat.