I don't think Riku ever "lost" his heart like Sora or Kairi did. As far as I am aware and from what I recall, Riku was always present in Xehanort's Heartless. Actually, I think it would be more appropriate to understand it as Riku being possessed by Xehanort's Heartless than Riku actually losing his heart.

:O They are called Unversed now? Interesting. It also gave me an idea! From what was said, I assume Unversed are predecessors of Heartless and Nobodies. They also feed on negative emotions. So maybe Heartless and Nobodies came to being when an Unversed "ate" (for the lack of better word) all the negative emotions of a given being. The Unversed, filled with the negative emotions becomes a Heartless while its prey, emptied of its negative emotions, becomes a Nobody? Though the only problem with this is what happened to the positive emotions? Unless somehow those are consumed when the Unversed becomes a Heartless since Heartless feeds on hearts. Anyways, just a theory that came to me. Not well thought out. :x

Maybe Kingdom Hearts is not a constant? It only arises when hearts start to gather. It always seemed to be something rather abstract given physical form than having any permanent, concrete existences of its own. :/