Board Thread:General Discussion/@comment-4041418-20140315151044/@comment-25223616-20140726004750

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.