The identity of the demon is unknown, but it is pretty conclusive that FSA Ganon is not the demon (it deliberately separates him from the demon).

As for the ambiguity in one line of text, which I explained in my previous post, there is one interpretation which befits the story and one interpretation which has no visible support in the story. Clearly we can deduce which interpretation is correct.

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What if Majora's Mask was never made? / The last interesting timeline topic on ZU /
Disproving the Divine Prank myth