Sensui's a very powerful ex-Reikai agent who decides to wreck havoc by breaking the shield between Makkai and Ningenkai so even powerful and deadly youkai can traveling freely between the two worlds.  He is a very extreme person who started off killing any youkai in his path.  Later on when he saw innocent youkai being tortured by humans, he cracked up and became the exact opposite and wanted to destroy the entire human race.  He also watched this "black" tape that shows all the cruel things that humans did over the years and that added to his hatred for humanity.

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, he 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!