We can rule out that unbirths are an early form of heartless if not only for the reason that unbirths existed before heartless did, We also know that they could not have been parents or ancestors (directly anyways) because heartless were created when Xehanort was experimenting on the Darkness in peoples hearts.
Xehanort didn't start doing this until just a little while before the events of KH1 started.
Which would have been approx 10 years after the events of BBS, when the unbirts existed
Thread=CLOSED
Xehanort didn't start doing this until just a little while before the events of KH1 started.
Which would have been approx 10 years after the events of BBS, when the unbirts existed
Thread=CLOSED