User blog comment:Arachne sama/Ch. 500 Jidanbou and Rin acting strange mystery/@comment-1586993-20130609024240/@comment-8669536-20130609192622

I think everyone wants a better explanation for this particular incident, but we'll just have to wait for Kubo to clarify.

It could certainly be something more interesting than a Quincy, but right now I think it probably is a Quincy. But it could be something like an Arrancar taken into the service of the Vandenreich.