Iain McElhinney will play Val-El, Seg’s grandfather.
McElhinney is an actor and director from Northern Ireland. He is best known for his roles in Derry Girls, Taggart, The Fall and as Barrister Selmy in Game of Thrones.
Aaron Pierre will play Dev-Em, a commander of the Kryptonian army.
Pierre has previously had roles in Britannia and The A Word.
There will also be appearances from guest stars including Sherlock’s Rupert Graves and Hollywood A-lister Paula Malcomson, who is best known for Ray Donovan and The Hunger Games.
Krypton premieres on E4 on Sunday, August 19 at 9pm