For some time now I have been reading the forums, and have come to the conclusion that all or them are connected. I know it kinda is a little far fetched, but take a look.
ES-------->Xehanort----------->Xemnas
| \______Xehanorts heartless
|_Riku
Sorry about the crudeness of the chart, so I'll explain it in detail. ES looses his heart to the darkness. This creates a heartless and a nobody, but they are recombined.
My reasoning for this is that Xehanort has the name of a nobody (no heart + x) but has a heart, so they must have recombined. This would also account for the loss of his memories, as his heart (where his memories are) must have been effected in some way.
We know the story about Xehanort and what he becomes, but Riku is unique. It is most likely that Riku is either the manifestation of ES's ability to wield the keyblade. Zexion also commented that Riku and Xemnas smelt the same, drawing some connection (both "descended" from ES).
ES-------->Xehanort----------->Xemnas
| \______Xehanorts heartless
|_Riku
Sorry about the crudeness of the chart, so I'll explain it in detail. ES looses his heart to the darkness. This creates a heartless and a nobody, but they are recombined.
My reasoning for this is that Xehanort has the name of a nobody (no heart + x) but has a heart, so they must have recombined. This would also account for the loss of his memories, as his heart (where his memories are) must have been effected in some way.
We know the story about Xehanort and what he becomes, but Riku is unique. It is most likely that Riku is either the manifestation of ES's ability to wield the keyblade. Zexion also commented that Riku and Xemnas smelt the same, drawing some connection (both "descended" from ES).