I didn't read over the whole list of helpful terms in the preceding posts, so I'll just try and make it right now as best I can.
Earlier tonight on Cartoon Network (US version) they showed an episode of Yuu Yuu Hakusho where they explained that the former Spirit Detective, Sensui, had split into seven distinct personalities. His lover, Itsuki (both are male) explained that as he was plotting to destroy the human world (ningenkai), Sensui had to increasingly split himself off so that he could do all that was required of him. I was surprised that they actually referred to the possibility of Itsuki and Sensui being lovers; although it's known in Japan, they glossed it over here by saying Itsuki was in love with Sensui's female alter.
One of Sensui's alters is a young girl, one is some violent angry guy, one is "Dark Angel" who was fronting presumably for most of the time the viewer has seen him before. They all have specific functions. Today Sensui himself fronted, and it's the first time anyone has seen him other than within the earlier stalking/chase scenes (when you didn't know that he was multiple).
Now, the portrayal of this character isn't so great, as you can probably tell, though I might buy these volumes of the set uncut so that I can actually get the story straight. :) The main protagonist (Yuusuke) actually openly calls Sensui a "freak". 'Course to someone like me, there's no reason to expect his character wouldn't do this. The background to why Sensui wants to destroy the human world: he came across some kind of indulgence party where humans were torturing demons. He had previously been helping to keep the boundary between the human and demon world intact but couldn't take what he saw, and went AWOL as the detective for the spirit world (reikai).
Of course...then there is Kurama/Youko Kurama, also. In the earlier tournament (it might be the Ankoku Bujuutsukai but I might be drawing that from Dragonball) the character "Kurama" aka Shuuichi Minamino was exposed to vapor from something called "Fruit of Past Life" which was supposed to turn him into a baby and make him easy to kill. Instead, it regressed him back to his former self as a cruel fox spirit by the name of Youko Kurama. (to make it clear, Youko Kurama incarnated from the demon world [makai] into Shuuichi's body in the human world in order to avoid being captured and killed.) The youko can't really appear at will however--this is a transformation, but with total personality change. I mention it because Yuusuke never called Kurama a "freak" (probably because Kurama was on his team). I seem to remember Kurama using "Fruit of Past Life" to change back into the Youko--this is all over the doujinshi, though. (doujinshi are comics done by amateur artists based on published work.)