User blog comment:Xilinoc/Ch. 542 - THE BLADE IS ME/@comment-11130627-20130621163942/@comment-11051098-20130621214022

Yes, from the little we saw, I think we're all sure that the mystic figure is Grimmjow. Also the way he spoke - or yelled - to Ichigo, that's another proof. I'm sure Kubo will confirm it soon enough.

He might even prove it in one of the very next chapters. We never know, he might switch to Hueco Mundo in 10 chapters, or maybe in the next one.