Kaepora the owl spoke of the land's guardian deity when he taught you the down left up song. Keeping with the parallel dimension theories: As Link was the defender of Hyrule, FD was the same for Termina.

Obviously, he died. Of all the theories I heard of how he died, I like this one the most:
A long time ago, the moon had decended upon Termina before. He sacrificed his life to stop it/or whatever evil entity that had caused it to fall.
But yeah, he died to protect the world, and where he fell, from his life-force, up sprang a giant world-tree. So yeah, that little theory would have him croking on the moon, which is a little far-fetched, but I like it anyways.

From that, you could say he was the one who originally felled Majora, turning it into a mask (a battle on the moon, mayhaps?). . .

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