Well, so much for wishing it was an all new enemy. It seems it's just a new pureblood. An awesome pureblood heartless if that, but not necessarily a new enemy for KH4 and beyond. Then again, we might get a better explanation on what it is in BBS V2. It might even still be alive, lurking in the shadows somehow, tracking Aqua, looking for another chance to strike. I wouldn't be surprised if it's a boss again in BBS V2. (Even if it Aqua actually did kill it, there could be more than one of these, just like Darksides.) I just hope it doesn't become another Vantias/ Repliku battle.
I still have some hope it'll turn out to be something of importance. Perhaps, it's just the beast/ heartless form of an ancient (evil) keyblade wielder, from the time of the keyblade wars. Maybe, he had nearly as much darkness in him, if not more than Xehanort, that he turned into that thing after some time in the Dark Realm. Hell, maybe the Dark Realm simply made him into that thing and he'll change back into human/ somewhat human in the Realm of Light, upon his release, just as Xehanort is defeated once and for all.