After reading alot of refrences in KH, KH:CoM and snippets here and there, I think I've figured out the deal with shells, hearts and whatnot. Feel free to comment on it.
In the world of Kingdom Hearts, a person (or being capable of intelligent thought) consists of three major parts: A body, a soul, and a heart.
Body- Simply put, the body is the vessel in which the body and soul are contained and sustained. Without it, you will surely perish.
Soul- This is a person's spirit. It is the embodiment of thought, personality and ambition/desire. It is the core of those things which make a person who they are.
Heart- Possibly the most complex part of a person, it consists of a person's emotions, inner strength and memories, and is both an anchor to the world and a powerful energy source. It also controlls your ability to distinguish good and evil.
However, it must be noted that the heart is not the single most important part of a person. If you lose your heart, your body and soul sent to a place between worlds, but your heart sinks into darkness, forgetting everything. You can continue living, but you are reduced to a life without emotion, without any memories but basic ones (aka, no memories from the heart). Without a heart, you are reduced to a half-life, consisting of merly a body and a soul. This half life is commonly refered to as a shell. If a heart is recovered, it is possible to revive a shell back into a normal being.
Since shells are lacking hearts, they know neither good nor evil, light nor dark. However, since their souls remain, they are often ambitious, or at least the ones we have seen were. As we already know, every member of the XIII Organisation are/were shells- Yes, even Axel. Also, to losely quote that same infamous member: "We're nobodies... when we die, we become nothing."
With this information, it is clear to see one simple thing- BHK is not Sora's shell!! To have a shell, Sora would have had to have lost his heart permamently. In saying shells are sent to the world-inbetween, I don't mean that normal beings with hearts cannot live there also. Ansem said himself that to wield a Keyblade, you must have a powerful heart. Therefore, BHK must have a heart. Ipso Factso.
I might add more later. Feel free to express your opinion.
