I don't recall any quote from Kaepora about a guardian of Termina (and he probably didn't mean the FD anyway), so all that's known of the Fierce Deity is that he's not an actual person. The mask never comes to life, and there is no mention of any Fierce Deity in the entire game (only the mask via gossip stone). The mask is completely optional, and the ending does not change if you have it or use it. Seeing as how Majora's Mask gives you the FDM (which is unreasonably idiotic), and that the FDM is supposedly more evil than Majora's Mask, I doubt that it actually ties into the story, seeing as how it would completely overshadow the title mask.

Seriously, if there was some god that lived in Termina before the Four Giants and was more evil than Majora's Mask, I doubt that they would be forgotten. Heck, they would probably wipe Termina off the map long before the Four Giants came. That would be a critical part of Termina's history, and would also help the game be more in-depth. Furthermore, how would Majora's Mask get the mask, and why would Majora's Mask give it to you?

Also, in the Japanese version, the FDM/MM thing was like a game of tag, meaning that the FD isn't necessarily a god (demon at best), and could just be some gift so you can play the "game." Regardless, the FDM being canon makes Majora's Mask the dumbest Zelda villain, and Link would have to be less intelligent than Majora's Mask to try it on. I doubt that the FDM is relevant to the plot, if even canon.

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