Itsuki is Sensui's close friend (some fanfics say that they're more than friends because he seems to like Sensui a LOT).  He is a youkai who can cut through dimensions and has this strange white hands around him that can shift dimensions.  When he first met Sensui, they were enemies but for a strange reason, they later became good friends.  He owns a pet (I don't know its Japanese name, sorry) that lives between two dimensions so whatever it swallows goes into another dimension.  After Sensui's death, Itsuki takes Sensui's body, and they stay in this other dimension where no one can bother them.

Seiyuu: I dunno... sowwy but if you do, tell me!