From my understanding of Bleach, it should be impossible for there to be a good-guy Espada, and for now at least it appears that Nell is a good-guy (okay, gal).
Espada are highly evolved Hollows. And a hollow has to feed on human souls to survive, and evolve (bad guy territory right there).
Regular Hollows become higher ranked after eating fellow hollows. This seems to me like a savage, merciless, dog-eat-dog existence. And a hollow has to do many cruel things to exist, much less to evolve.
In my eyes, only the most savage or deceptively clever Hollow can rise up the food chain, all the way to becoming an Espada.
Can someone please explain what kind of circumstances could allow how a good-guy Espada like Nell to exist in Hueco Mundo. Isn't this an inconsistency in the story?