I remember a theory (maybe even fact?) that the Espada all have markings somewhere on their face signifying that they're actually in there and not like the other weak Gillians.
But, I just looked through one of the old chapters and it turns out that Grand Fisher had some markings right under his eyes. Could he actually be Espada?
He was killed in one hit though... so he's too weak, or is Isshin too strong for him? Any thoughts?