I haven't read Gaiden, but I do know parts, and some of it was in the anime (no judging, it led to my getting the manga) but...isn't Nataku kinda comatose or something? Or at least mentally shut out?

Maybe a piece of Nataku(like his soul or some such) out and about in the human world for freedom or some such?

(And, of course, I could be remembering/thinking completely wrong...)