personally.. i think that one something happens to anyone's heart, a shell of that person is made, and that is what the unknowns are... maybe. Well, Tetsuya Nomura said that one of the Unknowns from Deep Dive is Riku. so that makes you think "what happened to riku to make him an unknown?" well, unless he just decided "i'm gunna wear this outfit to get past someting er other" i think it might be because Ansem took over him, and all that was left of Riku was his heart and soul, and since he wasn't darkness he couldn't come out of Kingdom Hearts. so i think maybe his Body was just a shell, and became an unkown.

Like the Unknown from Final Mix said "I am... a Mere Shell..." so that leads me to believe that when people lose their hearts their bodies become unknowns.

but that explains nothing because they obviously have awesome powers, so i really have no idea wtf they are, and i think about it a lot... and i got nothing.