im not sure if theres already a thread for this, but i thought up a theory while playing COM.
i was playign COM and Axel said soemthign that made me think "hmmm" and what i thought was that maybe all the unknown hooded characters, are old keyblade masters, and because sora wears black clothing in KH2, hes starting to loose his keybalde powers and is turning into an unknown, and thats where the BHK comes in to take his place with fresh new keyblade poweres being the white clothing.
anyways, thats just what i think, feel free to post your own theories