CONTAINS SOME SERIES SPOILERS!
I'm far too lazy for a full theory, but I've always thought that, prior to Terranort losing his heart to darkness, Master Xehanort himself was the byproduct of another similar transformation. Quite literally, Another. As we know, Xehanort's name has two Xanagrams (officially coining that term) that have been officially pertained to in the Kingdom Hearts universe. We have 'No Heart' seen as the name of an extra boss donning Keyblade Armor identical to that of Xemnas and wielding Master Xehanort's Keyblade. then we have 'Another', something seen entered into Ansem the Wise's (Ansem's the Wise? No...) computer by Xemnas as a password to the hidden Chamber of Repose.
Now, beyond those two facts the rest is speculation, but that's the fun part! I speculate that Master Xehanort is (okay, was) a Nobody. 'No Heart' may have been someone who lost their heart, becoming the first Heartless. Their body, then becoming the first Nobody is somehow given a new name, likely in a similar ritual to Xemnas' names for the member of Organization XIII. 'No Heart's Nobody, takes on the moniker 'Another', describing his connection to his counterpart. Note the wording used to describe a Nobody int the first research they were discussed in.
Where does the body go when it separates from the heart? If the soul remains within the body, is it still
considered to be deceased?
When the heart returns to the Heartless, the physical form disappears. But that is merely true in this world.
Perhaps the body exists in another form in another world.
If that is the case, then it is possible for one to exist in two worlds.
A being that is neither darkness nor light; belonging nowhere; abandoned by its heart; a mere shell of its
former self.
The relation between the heart and the body is complex. However, I am certain that if your self exists here,
then by definition the other cannot truly "exist." The other, the one which does not exist, shall be dubbed,
"Nobody."
So we have 'No Heart' and 'Another'. The latter of the two is essentially the prototype for the Master Xehanort we have all grown so fond of. After a while of experiencing his new existence, sans heart and purpose, 'Another' begins to gain his own identity and grow more self-aware. Yadda yadda yadda, he eventually sheds his purposeless shell of a name in some existential journey of self-discovery (likely including the discovery of some life-altering source of purpose - the χ-blade). In some sort of surely story-filled and epic way, the young man previously known as 'Another' finds his new heart's calling in the pursuit of this χ-blade and devotes his life to unlocking it's mysteries. He forges his new name. χehanort.
Now, how does this bring us to the events of Birth By Sleep? Well, somewhere along the line young Xehanort, in his obsession, crosses a line. Perhaps a similar thing is done to him as was done by him later to Ventus. Perhaps someone extracted his hearts darkness or otherwise locked it away inside his heart out of fear, rendering him comatose. With his heart in a state of slumber, and his master fearing it too dangerous to unseal the darkness in his heart, Xehanort is essentially exiled to Destiny Islands, and there remains in slumber... Until the heart of a boy reaches out and mends Xehanort's heart. A resident of Radiant Garden. Braig (Xehanort's future accomplice who might be the one who discovers the Chamber of Repose in the basement laboratories of Radiant Garden's castle and brings it to Xehanort's attention).
This brings us to a young Xehanort standing on the shore, where he receives a mysterious visit from a man in a brown robe.
So, recap and other stuff...
- A man loses his heart to darkness. Heartless = No Heart. Nobody = Another.
- Another grows a heart, becomes obsessed with χ-blade. Becomes Xehanort.
- Xehanort's obsession grows darkness in his heart. His darkness is sealed away, breaking his heart, rendering him comatose, and damaging his memory.
- Braig's heart mends Xehanort's.
- Perhaps -and quite likely- the Chamber of Waking can return a sleeping heart to it's body for awakening, in collaboration with the Chamber of Repose. Maybe after possessing Terra's body, Xehanort was able to access certain memories inside Eraqus' heart (as it resides in Terra too) and discover information of a second Chamber, residing in the former Land of Departure (or he already knew of it somewhat, since he had the same Master as Eraqus did). And after discovering hat Aqua holds the key to that place, he tries to use her armor to find her. Maybe Xehanort is trying to find a way to rejoin himself, 'Another', with 'No Heart'.
Oh, and
Guardian would be Xehanort's sealed darkness, released after Xehanort stabbed himself. Kind forgot what miy point was in writing this... Haha.