This is my fan-fiction/fan-theory based off of what I know from the Kingdom Hearts universe as well as my own speculation.

Kingdom Hearts facts noted:
-Organization XIII was founded in Birth By Sleep which takes place before KH1 (which is when the Naturespark Gem is obtained), and among the founding members was Xigbar.
-Bambi's homeworld was destroyed by Heartless but Bambi himself lives on in the Naturespark gem.
-The Naturespark Gem is obtained in The 100-Acre Wood world in KH1
-Bambi is still a fawn in KH1 so it happens before the second act of the film.

My speculation:
-The Prince of the Forest would have a strong enough Will to become a Nobody, as would Faline, the future princess.
-The heartless invasion of Bambi's world takes place at around the same time Bambi's mother was killed, and as such, Xigbar would take the place of Man.