I don't know...If Xehanort was Terra, why is he in Final Mix?
The boss is a lingering spirit, better concidered as Terra´s soul.
It is nothing more than an empty armor with a soul inside. Look the cutscene, you can´t see his mouth in the hole of the helmet, there is just darkness.
Terra´s heart and body went into Xehanort and his soul was left behind. MX did not have heart. When Terra and MX accidentally fused, the mixture was like this:
~
Terra´s young body +
MX´s (old) body +
MX´s soul +
Terra´s heart = Xehanort
So Xehanort(ATW´s apprentice) had two bodies inside him and MX´s soul plus Terra´s heart.
When Xehanort became heartless, he had human-form, because there was two bodies inside him. MX´s body went to the heartless and Terra´s body went to the nobody.
So now the heartless had Terra´s heart and MX´s body.
And the nobody had MX´s soul and Terra´s body.
Now when the heartless and the nobody have been destroyed, they would normally become whole again, BUT this case is different!! Terra and MX wont become Xehanort again! Terra will come back as a person with no soul. Master Xehanort will come back as his normal self. There you have it, the KH3!!!!
Any questions about this theory?