Board Thread:General Discussion/@comment-6057702-20130802173149/@comment-11051098-20130802210716

I dunno about the manga, but in the anime, Aaroniero told Rukia he was the only Gillian who was also a member of the Espada. I understood that he was still a Gillian.

He said that he gained Metastacia's ability of consuming the powers of every Hollow he devoured, and that this power made him the only Espada who could continue to evolve infinitely. He probably found a way to gain a human form and size due to Metastacia's or Kaien's powers, suppressing all the power he had into a human sized body, and that's probably how he became strong enough to become an Espada despite being a Gillian.

I think he also said that the form he gained upon releasing his Resurrectión, was a result of the 33.650 Hollows he had devoured. Sort of an army of 33.650 Hollows in 1 huge body.