It's worth noting that this Hollow doesn't behave like normal Hollows, it doesn't target those with high spiritual pressure, but specifically Shinigami. This might mean a few things, like:
- it has a grudge against Shinigami, which would make it a normal hollow of a former Quincy, seeing as Adjuchas and above level hollows have no longer memories of their past (they are agglomeration of 1000 of 1000 souls after all), unless it attacks Shinigami because of what Aizen, a Shinigami, did;
- it is under command of Aizen, but that doesn't really seem the case, it's seems that it was merely pointed to that location, rather than given specific orders, beside something that took down Ishin, would be able to take down Aizen as well (unless he used his magical sword);
- it targets Shinigami for the sole pleasure of hunting.
We need to remember that VL are highly evolved beings by themselves, they do not need to eat, drink, fight, they tend to seclude themselves in the deeps of HM. It would be strange for VL to randomly attack Shinigami in the ER, unless it is forced, like Espeda, or has other reasons.