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Could you elaborate as to what a ‘Mary Sue’ character is?

Mary Sues are basically female characters that have no flaws whatsoever.

Sure. It's basically an overpowered character.

Thanks! Might be something I like then

I would have to disagree with this comparison lol. Mary Sues (/Garu Stus) are characters that are literally so perfect that everything works out in their favor and they literally suffer from no true flaws. A good example (imo) is Tohru Honda from Fruits Basket. In that case, the whole of the Sohmas instantly like her upon meeting her (aside from ONE of the Sohmas who is shunned from doing so and ofc the main antagonist who I honestly care for more lmao) and she can do no wrong despite having a flat, boring and plain personality. Everyone loves her, everything goes her way, and there's only a few brief fleeting moments where she shows complexity (the only noteworthy scene being Kyo's transformation scene but other than that she doesn't show much complexity). In the case of this manga, Charlize is actually competent and has reasons for being competent (as it's said that all her hard work built up over the years transferred to her 13 year old self). Is she a Mary Sue? No. Is she OP? Oh hell yes. But there's a legitimate reason for that (as per the prologue) that explains why she's so damn good at swords and why she can use magic (though atm it seems like this magic doesn't have much use except to strengthen herself and make her conscious of her surroundings which play parts in the plot ofc).

Furthermore, Charlize has her limits: while she's good at getting people to like her through years of understanding social norms, she hits a wall at the 13th prince: a considerable point of conflict for her considering she can't just bat her eyes and he'd instantly open up to her the way a Mary Sue would have him do. She's also not infallible and is frankly human despite her OPness, faltering at the mother's kindness and struggling to understand the third prince (who she only knows one side of and is struggling to get him to open up that side to her, consoling herself by saying "time will cure it" when plenty of time has passed so far and he's still not there lmao.

As for the prince, while we don't know much about him yet, I'd say he's nowhere near being a Gary Stu. He APPEARS to be perfect and all-knowing, especially considering we know his true personality and how cunning he is, pulling strings and all, but he's naive in that he can't figure out Charlize and is cautious towards her, hitting the same roadblock she is in getting himself to understand her. He can't just smile and get her to reveal all her secrets and, even if he's curious, he doesn't want to ask her because that would mean showing he's invested in knowing who she is. He's just as flawed as the protagonist is, cunning in how he hides his true self but, as Charlize points out, he's not perfect at it (such as when he frowns when his tea is spilled, revealing his feelings quite clearly instead of masking them as well as he should).

If Charlize was a Mary Stu, the prince would've instantly opened up to her and already have fallen for her at this point in time (assuming he's the love interest, anyways). Her family would be begging to see her and apologizing for their wrongdoings, lavishing her with praise for their wrongdoings but are instead just confused that someone whose flattering attention on them suddenly stopped (which, I mean, if your best friend suddenly turned cold towards you one day without reason, wouldn't you feel the same?). Charlize would be able to find out who the assassin is from and enact revenge on that person and gotten the head librarian fired for annoying her, neither of which happened because she's still of low social standing and doesn't have much power (because that comes from progress and hard work lmao)

The prince isn't a Gary Stu because in his case Charlize would've instantly fallen for him and given herself up to him, telling him all her secrets and confiding in him even though they've only known each other for a few moments. His two-faced facade isn't perfect right now, either, and it's clear that there's a disconnect from the mastermind of the future's past and the childish boy who is hiding himself to make himself less of a target to royalty. He is smart, but he's also still young and immature and that shows in how he loves his mother (a trait that can be a downfall in a battle for the throne, especially if someone understands that killing his mother can potentially break his spirits for the battle for the throne) as well as how he can't control his emotions all the time, making him still learning and growing (and, as Charlize points out, there's only so much time left before she can get him under her sway and she's trying to do it as fast as possible but as soon as his mother dies, there's a big suggestion her plans would fall into ruin because he'd close himself off to her). I will say though that his ability of jack of all trades is sus but he seems like a typical prodigy character trope so im more willing to be forgiving on this, especially since it seems like he's a fast learner and can use this to his advantage to manipulate those around him.

Anyways I dislike when actually good characters are called Mary Sues so lol tldr they're not Mary Sues they're just stupidly OP w/ p much legitimate story reasons for being that way and flaws to boot. Also this story is at 12 chapters and we've just barely got started lmao. Just wait until the other princes show up and shit because then Charlize is going to have to hit more roadblocks that a Mary Sue could break through easily but she has to actually put some foresight into to pull off successfully

If you're reading this manhwa must mean you've read the novel beforehand. But by the last paragraph, it seems like you've not yet read the novel. If you've read the novel, you would've known how much of Mary Sue the duo is. I don't hate the duo whatsoever nor am I saying they are not good characters. I just wanted to point it out so the " Mary Sue hater's" wouldn't get triggered later. And yes, I'm aware of what a Mary Sue is and that is why I'm using it to describe the characters. In the future, everything will work on her favor. You'll understand what I mean later on or if you read the novel.

Must be so. Mary Sue MC's are my guilty pleasure.