Board Thread:General Discussion/@comment-4041418-20140315151044/@comment-4041418-20140726005628

Sithryis wrote: In my opinion, Yammy was #10 because Aizen couldn't figure him out. Aizen didn't want to make him high ranking but also knew he was strong so he put him at #10, the weakest Espada, because he didn't know how strong he was in that form, due to him either completely outmatching his opponents or looking weak. When he releases however, Aizen knows he COULD potentially be stronger then anything, so he made him #0 for that.

That, Sithryis, is the most likely answer for how Yammy ended up being ranked 10 AND 0. After all, considering how hard it must have been for Kenpachi and Byakuya to kill him once he unleashed his Resurrecion, Ira, shows Yammy was not as much of a pushover as he appeared due to sheer power and endurance thanks to all the spirit energy he'd need to consume to be at his max power, which it seems he was in Hueco Mundo due to wanting to kill everyone who was an enemy to Aizen.