Board Thread:Questions and Answers/@comment-10922879-20130617143901/@comment-11051098-20130622101859

I think we all agree that it is Grimmjow, there's no one else it can be.

Regarding Ulquiorra, he only said that not even Aizen had seen his second release form. He didn't actually say he didn't know about it. But I believe the Espada were ranked according to their reiatsu while in sealed state. It's possible that Ulquiorra was the strongest in terms of... pretty much everything.

Ichigo didn't manage to defeat him until Hollow Ichigo took over, but when that happened, he dominated him completely. By himself with his hollow mask, he didn't do much damage to the released Yammy, but he was probably weaker after the battle with Ulquiorra. And he couldn't use his hollow mask a second time to defeat Yammy, probably due to the outburst of Hollow Ichigo on the roof.

But I still think Ulquiorra was stronger than Yammy anyway. He had great speed and power, and his Cero Oscuras and Lanza del Relámpago. Yammy only seemed to have a greatly improved physical power and strength.

And Ulquiorra was indeed the kind of person who followed every order to the last letter, without protesting or hesitating. I think that's why Aizen trusted him enough to let him lead the first invasion to the world of the living, and to lead the second invasion, threatening Orihime to come to Hueco Mundo without telling everyone, while Luppi's team distracted Hitsugaya's team.