Its seems that throughout kh there is 3 factions, light, between, dark. What if in the keyblade wars that the 3 factions started too conflict and it caused a giant war? If so it seems that darkness was destroyed in the war, seeing as how there are very little of them in the current games, and without light and dark maybe the between dissapeared as well? So light was dominant. But you cannot have light without dark, so maybe in bbs when MX says he made a mistake maybe that was trying to create a new balance which by doing so created the heartless and nobodies. So then darkness took over as the new balance and wiped out the rest of the keyblade wielders. But to counter act what has happened with the balance, maybe a between was born as the chosen wielder to set back the balance, Sora, the between. Sora was about light and yet he did fall into darkness but didn't become absorbed my it. Also maybe thats why he can duel wield as well?Now looking back to bbs, Terra falls into darkness, lust for power basically it seems. His fighting style is all about power so it makes sense. Aqua is light. She has many attacks that are light base. A rainbow blast and other asortments of things like that. Also her way of expression just screams light. Ven the between. Think about it why is his clothes half black and white? Makes sense doesnt it? His powers are like sora's. A blend of light and dark. The speed attacks (light based moves) mixed with physical or power attacks ( dark based moves) It makes sense. Look at the current keyblade wierlders, Riku darkness, Sora between, Mickey light ( or kairi we havent really seen anything about her yet, though).Please do not flame me this is just speculation.