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Only Marluxia and the others are Nobodies so you can't really compare. Donald, Goofy, Sora and Jiminy all lost their Memories. Argualy, Riku did quite a bit of his own too, and Mickey wasn't there long enough to be truly affected. All of those - are whole beings.It's Namine. She makes all those fake memories. And if it were CO, then Marluxia and the others probably would have lost their memories.
I think CO is the one that turns memories into worlds.