By Ragnarok.Raenil 2012-07-10 22:25:15
My first thought was Aizen, given Urahara's reaction.
Halibel, who was stronger than Grimmjow but weaker than Urahara, was beaten by Kirge. So unless he's gotten significantly more powerful than them in two-ish years, then there's no way he'd have beaten Kirge, who easily took down both of them.
Also, if that's an Arrancar, they haven't used their Resurrección, and it took Ichigo in Bankai to beat Kirge.
It's likely a Shinigami and to the best of my knowledge Urahara and Aizen are the only ones who can make a Garganta without assistance from Division 12.
How Aizen would manage to unseal himself is beyond me though.