A "Nomura" XD;;;;
Some say Repliku did eat up Riku's Memories, and that was why his Memory worlds were so empty. I relate it more to him not having Namine to create the Memory cards, and think Diz used either those cards themselves or replicas of them. As these would essentially be Sora's Memories, Riku wouldn't have much to relate to in them, and the ownership disonance itself would explain the emptiness - aside, of course, from the people the both of them met and related to Riku's Darkness, being the villains.
Either that, or I'd sooner relate his Memories disappearing to Xion as well. Repliku only had Riku's Memories because Namine copied them into him and she admitted later to not having messed with them when Riku refused to go to sleep. Seeing how Repliku at least seems to work in a slightly different manner from Xion, I think if they really did go missing, Xion ate them up too.
The day she revealed her face was, after all, the very same day Riku showed up in Castle Oblivion. That's a Nomura if ever I saw one.