Kokuyou, unlike Fuuma in X, is a supporting player in Wish. He's the Prince of Hell, and the lover of the angel Hisui. Being technically a demon, Kokuyou is supposed to be evil... much like Fuuma is assumed to be evil because he's the kamui of the Dragons of Earth. Also like Fuuma, it turns out that Kokuyou is not the "big bad" of Wish... unlike Fuuma, we find this out almost inmediately. And he doesn't ever actually hurt anyone. Or at least, not that we see.
Physically, Kokuyou is another character that is modelled on the same basic design as Fuuma: exact same hairstyle, body build, and similar eyebrows. Tends to wear dark clothes, sometimes a long cloak reminiscent of Fuuma's coats. Unlike Fuuma, he has earrings in one ear. The most striking physical difference with Fuuma, is that Kokuyou is missing an eye... a trait which he shares instead with Sakurazuka Seishirou (who is actually Fuuma's antithesis in many ways).
Something that is not a similarity in looks or personality, but that I find to be an amusing parallel to Fuuma, is Kokuyou's lover Hisui. Hisui is an angel, slender, delicate, and ethereal, with long flowing blonde hair. Despite being a powerful angel, one gets the impression, looking at him and Kokuyou, that Kokuyou is the protector... and in yaoi terms, Kokuyou is a tall dark seme, while Hisui is a pale, feminine uke. The obvious parallel to Kokuyou/Hisui in X is Fuuma/Kakyou. Fuuma is the tall, dark seme, at once possesively dominant and tender and protective of the frail, beautiful, blonde Kakyou. Who, by the way, is quite feminine... even if not as much as Hisui.
And yes, I do ship Fuuma/Kakyou, as a matter of fact. =D