**POSSIBLE BIG SPOILERS**
As I posted before in the comments (before seeing this topic) Marube can't be Shibanmushi, as Marube is still trying to break Rika sexually, but Shibanmushi told her he/she wouldn't see her as a woman.
Also, Shibanmushi's purpose was to prove her that Amano wouldn't be able to see her as a man, not that he'd sell her out (as Marube just tricked everyone into thinking).
Their actions seem to overlap, but their intentions are different.
That said, no other character seems to fit Shibanmushi, with the exception of Inspector Yamashina.
If you all remember, Rika stabbed Shibanmushi in the abdomen when fighting in the maze under Ghost Tower, but while bathing Marube, he shows no signs of an injury or scar.
But then, in chapter 35 we see this revealing conversation:
So, I'm calling it now: Shibanmushi is Yamashina!!
Edited by Kurono, 10 February 2014 - 10:36 AM.