I didn't say all of his memories. It is clear that even though he didn't know who his true nature was, it was enough to illicit a strong reaction from him; he had to have known something. Furthermore, if he didn't truly know anything at all, there would be no reason for him to join OXIII, and there would have been no inclination to. He can stay perfectly ignorant, but, that didn't happen, so he had to have known something and wanted to know more.
His frustration with Riku probably came as a result of the natural Nobody disposition to always be pissed, and for Riku challenging his identity. I would be pissed if someone said that who I was did not matter.