How would you explain this in bbs.
I'm pretty sure the question of "where does a person's heart go when they die" isn't really going to be dealt with in the series at all, because it'd cause more problems than it's worth.
Whether or not the "Heart of Worlds" Kingdom Hearts gets explained depends on whether there's a character in the position to study it, so it might remain as something that's only explained in interviews, too.
The heart holds the personality hmmm... I would think the soul would have the personally.
Normally, that's the case - but, Sora and Riku both had an "existence of only the heart" at certain points due to the loss of their body and soul, and both kept their personalities.
The Princesses of Heart, on the other hand, are our only example of body-and-soul lacking a heart where there are no traces of the heart left over to form a personality (as is the case with a Nobody), and they essentially turned into vegetables. They could breathe, but that was about it.
okay... but we mostly referr the spirit to the soul.
Like I said, I chose the wrong word anyway.
What the KH version of the soul seems the closest to is the reflexive, body-oriented aspects of a being. It keeps the person breathing, and therefore alive, but the heart is what makes them a person.
Basically, if a work of fiction suggests that there's an afterlife to ease the passing of a sympathetic character, and this is not contradicted elsewhere, it's usually safe to assume that said afterlife exists.
Since nothing in canon contradicts the idea that the Riku Replica's heart won't simply vanish, the implication is that hearts do go somewhere (and that the heart, rather than the soul, is the thing that one should be worried about).
As for my thoughts about potential connections to the Unbirths - I think that, if they're related to the soul at all, they are beings who
never had souls to begin with and are therefore, in a sense, unborn. Or, on the other hand, if we take "Unbirth" as "a being whose birth was reversed," I suppose they could be existences of pure soul (if one considers birth as imparting a body and heart).
...I'm actually inclined to think that the second definition of "Unbirth" is more likely, though also much harder to predict, since having one's birth reversed is not the same thing as dying.