Another member of the Inhumans royal family has been cast: Ken Leung (Lost, The Night Shift) has been added as Karnak, the cousin and closest advisor to Black Bolt (Anson Mount).
From ABC: His gift is an ability to see the fault in all things people, plans, structures, everything. Hes Black Bolts most ardent supporter and strategist and is often seen as the royal familys planner and philosopher.