You seriously make it sound like you've played the game beginning to end and seen everything Sera has to offer.
I would wait a little while before jumping that far if I were you.
Not once have I said "Sera's a Mary Sue".
I've said her listed traits and what we know make me think she could end up like one. It's purely opinion, non-concrete opinion, one that can change. It's just opinionated worry from me about her.
The "light in the dark" line apparently had nothing to do with her, but a different character with a similar name (why that was quote was posted, then, I have no idea), so that definitely makes me less worried. But even still, I just don't like Sera so far.
I think a handful of you Sera fans should stop being so defensive when I or others have tossed critique to her. I've not, nor will I, tell you to "hold your judgments" about positive remarks on Sera. I could just as well say you don't know anything about her, no more than I do, and she may not end up being a good character. But I've not done that. It's all a matter of opinion and perception on what we know of her and what's been said; people can take it all a number of different ways, and no way is wrong, since it's all purely an objective opinion (assuming you can back up said opinion logically).
What one considers a good character may be absolutely hated by someone else.