The Warrior of Light: Plot Device, Mary Sue or A legitamate charatcer

[Risvertasashi]

....Not even gonna argue Minfilia (I hate her) but:

Moenbryda: Out of nowhere? No. We know where she comes from. And she had a history with Urianger. I wouldn't say 'loved' by the others, just accepted because of said scholarly origins (Actual plot background explaining it! Not something a Sue would have). She was pretty much a plot device, yeah the sacrifice was a bit schlocky, but Mary Sue is a stretch. One of the more accepted definitions of Mary Sue is that all their specialness isn't developed by plot and is there inexplicably. Big fail on this count.
G'raha: Better than the rest of us? Other than that apparently even Allagan royalty can't resist catboys... there's not that much notable about him. Another plot device. Some poor writing with the conclusion, to be sure. Mary Sue? More like one-trick pony.
Ysayle: Well Estinien & Ishgard as a whole isn't so accepting. WoL, on the other hand, seems to readily accept heel-face-turns from anyone. There's a fair bit of plot around her rebels and summoning Shiva because they idealize what they think she stood for. Powerful yes, but again there's plot/background/lore to explain it. And Heavensward plot in particular develops in the direction of how it's not all she thought it was.
Warrior of Light: The "shows up out of nowhere" is just a narrative tool to allow players to headcanon their own story. Not loved by everyone at first; s/he has to work their way up and earn it. Again, plot/story development here. And even then there's plenty that resent you (including certain Syndicate that try to frame you!), although the story does take the 'love the WoL' part a bit far. Plot armor is pretty much standard; protag has to make it to the end, and MMOs don't have a definite end.

Now I do think you point out some good issues. I just don't think "Mary Sue" (or "Canon Sue" or similar terms) is the right label. (Although one certainly can stretch it to fit anything).