Triton’s brother, Karnak, is one of the most interesting characters in the Inhumans because he’s not actually an Inhuman. He’s one man with no actual superpowers, but with a knack for puzzle-solving and martial arts that allow him to pinpoint weaknesses. He’s also described as very wise and usually counseling the king.
His position in the royal council makes me think of a seasoned character, which calls for a seasoned actor like Isaiah Washington. I’m aware he and Adam Scott would have to play siblings and there’s race difference here, but in a world where one would be green and with gills, I think a color-blind casting would be acceptable. Washington’s credits include The 100, Bionic Woman, and he played Dr. Preston Burke in Grey’s Anatomy (an ABC medical drama). Now, at 53, I’m not so sure how keen he would be to the martial arts, but that’s what stunt doubles are for.