I'll admit, I never really figured out the relatonship between Seija, Tojiko, Futo, Miko, and Hijiri. That whole aspect of Japanese culture is just totally opaque to me.
...I'd say that has rather less to do with "Japanese culture" than "ZUN being ZUN", ie. maddeningly vague just because he can (and cuz it leaves the fans leeway to fill in the blanks as they wish). Plus Seiga's Chinese anyway :U