Unversed means "Those who are not well versed in their own existence" and according to the Kingdom Heart's wiki they only "grow off of negative emotion" which I might interpret as meaning they are drawn and grow from the emotions of people. They are also suppose to be the "opposite of life".

I would guess they are possible the original form of the Heartless and Nobodies. By that, I mean a creature spawned from darkness that may have lost their heart but did not go through the process of splitting into a Heartless and Nobody rather it being a Heartless that may have kept their body however warped.

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Here's another debate I've pondered with people is whether or not Riku has a Nobody. He was possessed by the Heartless Ansem but its never stated if Ansem consumed Riku's heart like a normal Heartless. Do you think Riku qualified for a Nobody? Is it possible he might have a special nobody like Roxas or Namine? Just a simply debate I got into and sense we're kinda discussing unexplained phenomena already its fine to debate.