The answer the girl gave is actually way more correct than the question.

The right question would be "Qual'è il problema?", while in the answer the only correction I would make is to remove the comma.

And... Wow. The US didn't exist until, what, three or four centuries AFTER 1470? Hell, nobody even KNEW that there was a whole continent there until more than thwenty years later!

You've got a really good point actually. Calling her an outright Mary Sue wouldn't be fair. More like she's walking a tightrope. She actually does have powers that don't exist at all in canon proper. Moreover, she was able to take down a creature that all the other characters in canon proper still haven't been able to fully overcome with all of their strength combined. Honestly its debatable if her initial existence (her and her tribe) violates canon outright because the author does make crafty use of a, ah...loophole in canon. I suppose you could write it off as a speculative AU so no harm done there.

Yeah the characterizations seem to be the author's strong point. The only thing that really bothered me was the lack of consistency between some of her actions and her personality way down the road in the fic. For instance from the first few chapters you get the feel that this is the sort of person who's excellent at short term, on the spot thinking, but doesn't have the patience or temperance for long term strategy, and she doesn't. Mostly she leaves all that work to her brother, but in the recent chapter she'd been revealed as the mastermind behind everything? Once again, a bit of a stretch.

I didn't mean to apply the label lightly and I should've provided more context for my descriptions in my previous post in order to avoid giving that impression. There's actually a lot of ridiculously strong and beautiful and well rounded female characters with tragic pasts in original fiction that I love. Like Shana from Shakugan no Shana. I honestly don't believe that an OC from an orginal work of fiction can be a Mary Sue, per se, because it's their world and their story. They are a part of canon.

OC's in fanfiction however, I've always been wary of. Not every OC in fanfic are Mary Sues. In fact I've seen a few that are quite the contrary and that have made me genuinely care about them. (I know this one in FMA fandom that's literally a girl after my own heart.)

Perhaps though, I've had it wrong about this character. She has some suspicious traits to her but calling her a Mary Sue might be a mistake because although she's not the most smartly written OC I've seen, I don't think she's one dimensional either.