So I talked with a friend about this earlier, and she pointed out that the avatar contrasts Byleth in a lot of ways. 

Byleth is usually compared with a demon and named after a King of Hell (Beleth), wearing black in their normal outfits, and normally depicted using swords. In comparison, this protagonist wears white, and was shown coming from the sky with one wing. It’s hard to tell in this picture, but their preferred weapon seems to be the lance, which in FE terms, beats swords.

So, taking this into account, if Byleth is supposed to be a demon (Beleth), this person is supposed to be an angel - more notably, Saint Michael.

As for descending from the sky with one wing, this could also be in reference to Abezethibou, who, in the Testament of Solomon, fell from Heaven, and as a result, only had one working wing - a result of following Beelzebub.

Byleth was always referred to as a demon, barely showed any emotions, but deep down was a good person. So, it could also be possible that this avatar is more of an antagonist in the story. 

Another thing worth noting is how similar their clothing looks to Arval. Those Who Slither in the Dark had an angelic looking person before, and Agartha did have its own religion - one we just never learned about. There might be some explanation for it in the game, but we won’t know until September.

Fortune’s Weave Avatar Theory