Jisen Meizuki
And I agree with you. That when it comes to Heartless and Nobody, it should be based on the storyplot, background, and history; not the characteristics or personalities of the characters. Many people thinks that Riku did become a Heartless because he wears dark clothes, but that is not true. Because if Riku did become a Heartless, he would be in the same state as Sora: becoming a Shadow or any of the Heartless. But he didn't, so therefore he doesn't have a Nobody.
It was even stated in the game that Riku just turned to darkness. It was never mentioned that he became a heartless and then turned into a nobody.
He never had his heart taken for him to turn into a nobody. So yes, definetly he could not have been any of the following based on plot and history.
I actually never came across people who agreed with that theory.