I think your points are interesting, but I feel some of your conclusions go a bit too far and reflect your personal biases more than the story itself. Here’s my perspective:
Story centered on the MC?
Yes, the narrative revolves around the protagonist, since she is the narrator, but that doesn’t automatically make it bad writing or misogynistic. Her life is far from perfect, and not everything goes her way. For example, her interactions with Eclis show that she makes mistakes and acknowledges them. He still acts according to his own choices, and we can’t blame the MC for all consequences.
Winter is portayed as a villain?
Winter was never portrayed as a villain. The MC may disapprove of his choices, but the story doesn’t present him as evil. We don’t always have to share the protagonist’s perspective, our interpretation can differ from hers. This is an example of how reading a story through only one lens can make it seem biased.
More Female characters?
I agree that more development for other female characters would have been nice. Most side characters only appear in relation to the MC, which is a limitation. Still, this doesn’t make the writing inherently bad, it could just be improved.
Classism and “villainous slaves”?
I’m not sure what you mean by classism here. The ex-slave as (sub)-villain is not a universal trope; in many ISEKAI stories, antagonistic characters are more ofter the kind (two faced) OG. Saying it “always” happens seems exaggerated and reflects a particular perspective more than a factual rule.
Does MC portrays a Mary Sue?
Not exactly. She’s portrayed as a victim who sometimes fights back, but this doesn’t make the story poor, it’s part of the narrative structure. Stories with a central character facing constant challenges can still explore emotions and psychology effectively, which this one does well.
One last thing,
This manhwa originated from a light novel around 10 years ago. Judging it solely by nowadays standards of morality or social critique may not be entirely fair. The story reflects human flaws and choices I believe, which is a strength rather than a flaw.
In short, while your points could improve the story, I think some of your criticism reflects your personal interpretation more than the text itself. The narrative focuses on one perspective, but it doesn’t force the reader to agree with the MC or endorse her actions. The story explores complex emotions and consequences, even if imperfectly executed. Anyway, it’s completely understandable if you choose to stop reading, since the story doesn’t align with your current moral perspective.
I can see the points that you're making, and I even agree on a few of them, but at the same time, I feel like you're judging too harshly while missing some of the finer details. (Because I can't remember whether we got MC's name in the modern world or not, I'll just refer to her as MC and original Penelope as Penelope)
The story is centered around the ml's in her life not just bc it's based on an otome game but also because the original Penelope was a commoner who got adopted by the duke because of her striking similarity to Yvonne. Because she was adopted and also practically hated by her family, Penelope couldn't make any friends and barely ever left the house before she was possessed. I mean, even her maid who's her closest ally now in the current chapters used to stick her with needles and feed her rotten food. So the reason why we only see Penelope hang out around men is because she only recently got the privilege of leaving the house again. While I do agree that having at least one other strong female character would be a refreshing change, it would also take away from the story of Penelope's life being tragic that the author is pushing.
While I was on the Winter train for a long time, Penelope and MC both had reasons to dislike Winter in general. I mean, he's the one who brings Yvonne back to high society in the first place. Not to mention, he's also the first one to protect Yvonne and bring her status up in the original game, you can't fault MC for being wary of him. All things considering, MC isn't in the wrong for being cautious around any of the men, as she knows from previous experiences (attempting hard mode in the game so long that she pulls an all-nighter while doing so) that any of them will kill her at any given chance. So when Winter thinks she's one of the Leila? MC has no choice but to think he's against her, because everyone's against her.
I also think making Eclis the villain was a bad move, but from a realistic standpoint it's also the easiest move for Yvonne to make. I think the reason why the ml is the Crown prince is bc he's the hardest for Yvonne to get to. Winter is a traveling sorcerer, so he could easily have encountered Yvonne outside of the capital. Derrick is Yvonne's eldest brother and obviously went everywhere he could to find his real little sister. Reynold isn't as keen on looking for Yvonne, but he's the second son of a duke and doesn't have as many duties, so he's able to go out (not that he ever really speaks to Yvonne, as he's accepted Penelope as his sister). Meanwhile, Callisto is the Crown Prince who's always at the battlefield and would never interract with anyone that isn't a soldier except for when he finally meets MC. But Eclis? Eclis is a slave who doesn't have a say in who he wants to speak to, or what he wants to do. If I was Yvonne? I would also go for Eclis, since he's also clearly in a bad place mentally even before Yvonne gets to him.
I do think that MC was wrong for leading Eclis on, and I wish that the story could've gone in a different direction. But I also feel like you might be a little overzealous in saying that the slaves are often the villains? Not that you're outright wrong since a lot of stories use slaves for uprisings and destroying of the royal system, but saying that slave characters are always tools for villainy is a bit of a stretch. Eclis himself isn't innocent either. In the beginning, all he wanted to do was kill MC so he could be free of his chains. But when he gains feelings for MC? He's quick to kill off even his own people just to gain status. He's manipulative even before he meets Yvonne and only becomes more so after meeting her. I feel like people forget that Eclis wasn't a slave from birth, but was the illegitimate son of the fallen kingdom's king, so clearly he used to be a royal. Unless Delman used to be a kingdom that forbid the usage of slaves, Eclis had to have been aware of the slave system and even supported it until he became one.
I also feel like calling the classism card is a bit unfair, again because MC was hated in her own household. How could she fight against classism and slavery if she couldn't even hold a spot in her own home? From the beginning, MC knew that Eclis would become her knight and wanted to buy Eclis to speed up the knighthood process, even going so far as to ask her father to clear Eclis of his slave hood. Advocating for the abolishment of slavery just because she came from a modern world would require a level of popularity and power that MC couldn't attain, considering she literally used to be called the "Mad Dog" of high society. To add to my point, MC also came from a modern world and still thinks that she's inside of a game, so from her point of view, none of the characters are real anyways so it's not like she's actually changing their lives.
Penelope and the MC are victims, like actually. The very first scene we have of MC in Penelope's body is of her being served rotten food and having multiple wounds on her arms. Even the butler is quick to disrespect her even though she's technically serving as the Duke's daughter. Yes, Penelope might have been crazy before MC took over her body, but she was a girl living on the streets before the Duke took her in all of a sudden just to shut her out once she didn't live up to his expectations. In the beginning, MC plays the game and has to start over again and again because everyone hates the villainess no matter what choice she makes. The game is on hard mode and specifically designed for Penelope/MC to fail. Not a single person was on her side from the beginning, so why shouldn't she be the victim in her own story? Penelope made one mistake with Reynold when they were children and he held that hatred all the way up to adulthood until MC finally cleared up the misunderstanding. Derrick hated Penelope and would always find a reason to put her down or villainize her even when it was clear she didn't do anything wrong. Callisto literally threatens to kill her in their first meeting and Winter is the catalyst for Penelope's death, being the one that brings Yvonne back. The reason why Eclis is the only one in the original game to stay neutral when talking about Penelope is because he's the only one that saw what Penelope had to go through at home and the blatant prejudice/disrespect she got everywhere she went.
TLDR: You've made some good points and I wish the story could've been done a little differently, but I also feel like you're judging too quickly while overlooking/diminishing a lot of the details that makes this story make sense. Or I could also be reading through rose-tinted glasses because this story is one I've been reading since I was young. Feel free to drop it though! I respect your opinion and it was really interesting to hear your point of view on this story.
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I see what you mean, my rant was fairly far fetched and influenced by how I just awoke to the insane internalised misogyny in webtoon (as in the app) specifically in villainess manhwas.
My point on the story being centered around Penelope I don't think were on exactly the same page on that, I just meant it feels like she has a shield of morality around her that makes her seem too perfect and makes it hard for me as a reader to be able to immerse myself in the story and see the characters as less one dimensional.
Winter isn't exactly portrayed as a villain, I commented about that a while ago but its the way SHE acts towards him that got me annoyed her perspective was overly critical of winter and it was perplexing cos like yo he's a normal guy, most would do the same if they were him. In my words "I just don't get why she doesn't see what's right in front of her" - comment made around 2 or 3 months ago about that situation
Ab9ur the female characters I am very woke and incredibly feminist, I can't enjoy a story if I see even the slightest undertone of sexism, hence why I can't watch/read shit like attack on Titan. No matter how good a story is, the limitation of female characters knocks it down many pegs in my book.
By classism ur right, I was projecting a lil, I'm just like baffled by how dismissive they are of commoner/slave characters, and baffled by how (sorry for the spoiler) in the novel apparently when she became queen she did absolutely nothing 5o address the slavery issue. It's just like???? And you know if she was a slave in this story slavery wouldve been touched on by chapter 12, and it's that lack or empathy that set me off.
Id agree MC isn't a Mary Sue! And you're right, I was just once again talking about her shield of morality, and how when the wronged and revenge trope is used too often it starts to feel shallow, and like a revenge fantasy and really snaps me out of the story.
Didnt know the story was 10 years ago, but I do believe I would've had the same views though less extreme 10 years ago, yes i was six, but these are base points I feel very strongly about and I don't believe in morals being influenced by the time you live in, because morals shouldnt be influenced by trend. Thats what makes us devolve.
All in all I agree with you a lot, I was rambling a bit because I'm just so frustrated by the lack of good stories nowadays, and this is a lot of MY perspective personally and my experience with this story, and to be fair, I am overly critical, but that's due to the fact that Ive been into writing and storymaking since I popped out the womb. If any of what I said comes across/came across as hateful, I will apologise now, thank you for having a logical convo with me!!! (≧∀≦)

So glad people are finally realizing these stories all follow the same formula of subtle misogyny, very unsubtle classism, and Mary Sue female leads that can do no wrong.
I hate how male centered this story is, how her very life revolves around men and I get that's the point because yk otome but how is there not a singular powerful female character apart from Yvonne???? Deadass??? She has no female relatives and the only women in this manhwa are either insignificant, dont have astounding appearances, dont get male attention or are evil if they do. Hello??? Not to mention the men that don't bow and scrape at her feet are also portrayed as evil; take winter. I'm not sure about now but in the earlier stages of this manhwa he was very nice yet she acted like he was evil because he wasn't willing to put his clan and family on the line for her, girl he doesn't know you!!!
The classism too is egregious! Villainising the slave character? You were the only light in a life where he drowned in darkness Penelope, fuckint obviously he'd become obsessed. And why does the tool for villainy ALWAYS have to be a slave?? What absolute bullshit. If you're reincarnated with modern values and you do nothing to advocate for modern values, you are subconsciously in support of the old time values you live in, and she can do that easily because she wasn't born a commoner. It's absolutely privileged ignorance and it's fucking insane.
Onto the Mary Sue point, she isn't exactly a Mary Sue but the fact that the author tries making it impossible for her to be the villain in any scenario that might make her look bad makes this story so fucking lackluster. She is always a victim, and when she isnt a victim, it's because shes "ooh being a badass and fighting back!" It's a constant cycle of people hurting her, and her getting revenge and it's an annoying fantasy where the world revolves around her, which, fairs, shes the MC. But also! That just makes the writing BAD. stories where the main character is invincible serve not to be an immersive, good story, but to be a quick shot of dopamine due to the constant cycle of abuse and revenge. I don't know when the writing took such a sharp turn into being ass, but I'm genuinely so sick of pretending this shit is good because it's not. It's not, and it's pretentiously problematic.