This Webtoon really has only one flaw in my book

It's so annoying how harem-focused this is. And yes, that's what it is. The MC cannot be without a THOT for a single scene. If there is none organically, from the existing cast, there will swiftly be a new one introduced.
Male characters are either capable but barely existent, male adult side characters as "role models" who appear rarely, or they are simply meant to be running jokes. Mostly, in stories like this, the female characters, who are literally meant to be bimbos in the first place, are getting their anus glazed every scene they are in. After all, the author tries to give them all "the same" amount of attention, because we need to love them as much as the author does, which is hard, because harem isn't always favored. In fact, it invites criticism from everyone who actually came for a story.
Believe it or not, most teenagers, in this genre mostly male, do have a phase in which they are thirsty as fuck, and some stay that way - but most just grow out of that phase pretty quickly and either turn to porn when they are horny or just want to read a story.
While I get that there are enough thirsty people, I don't get why you would ruin a perfectly constructed story like this. The lackluster female characters that only exist to be female will ultimately never matter, because they streamline a single girl for FL; that's pretty obvious. They are also all kids, so they don't have fan service (thankfully), and the protagonist is actually an adult at heart, not still acting like a 12-year-old around girls (thankfully), so all of that isn't even made for the thirsty ones. It's just there for no one, and I keep getting irritated at that fact, because it's dragging the quality down a notch.
With this, none of the introduced bimbos have a character, but they also have more screentime than other characters. If a male character is introduced as a support, they don't usually get the limelight to this extent and remain forgettable. Nobody pretends they aren't somewhat forgettable either. But harem-focused works like this pretend with their narrative that these throwaways are actually important and not totally forgettable trash.
There's more character in the dude who managed to get into Lumerne, while not being able to do much in fighting and shouldn't even be there, than in any of the added in female randoms, like that bitchy kid who ranted to MC because a mythical being decided not to hand her child to her but MC instead or that other bimbo who appeared for a single arc during his fun outing to the beastmen hero academy. Even among the heroes, the females are more important. Aron was a well-known hero, yes, but a lot less revered in human circles. Sure, Luna did magic, which is more important to humans, but why was Lysinas so great for humans, but Aron isn't as important?
But that's the smallest issue. The guy whose name I forgot because he barely ever shows up at all and nobody talks about him is someone nobody ever mentions, and his personality regarding arts made him a pervert and laughing stock most of the time (the blacksmith). Aron was a coward. But Lysinas and Luna? They are beyond reproach, and 80 % of mentions by the protagonist will be about Lysinas and Luna, not Aron and the Blacksmith.
This is the same as knowing that a bimbo will never be killed. Because they are meant to be collected and stacked like the Tower of Babel. Only males will be killed for purposes of plot and character development (unless it's about female characters that have already died to begin with, in order to set the protagonist on the path of grief and vengeance or whatever).

Actually, it's not the only flaw, but the other flaw is kind of minor, so I scratched it in passing. But I just decided to actually go into that as well. I just mentioned how Karl wasn't supposed to actually be a student at Lumerne, and there's a reason why I say that.
I like Karl. I really do. But this is a bit like HeroAca. For the 5 people lounging on a site like MGG who somehow don't know what that is about and why I start comparing something here: A bit like this story, where there's different Academies to train possible future heroes, HeroAca also has, like its name suggests, a plot with people who have superpowers and a world in which these kids can go to certain academies to learn how to be a hero with their abilities.
Like Lumerne, the greatest of them is the academy the MC enters at the beginning. And like Lumerne, they also have a teacher who threatens to yeet them all out on day one (just wanted to mention that). Anyway, I actually don't properly remember if HeroAca has many academies, but I'm pretty sure there are other academies that produce heroes. But it's better to compare them, because here, Karl actually entered a class that only had the bottom feeders in it. MC of HeroAca gets into the class with the greatest prospects, so the difference is kinda big on this one. On the other hand, here, Lumerne is clearly hailed as the greatest academy for heroes in the entire human realm. So there's that. Getting in there should be nigh impossible, just like HeroAca's Class A.
And just like Karl here, there are not one, but actually several characters in HeroAca's super-academy Class A who are literally not strong enough to even get into this school in the first place, let alone the spearhead class. I haven't watched or read HeroAca in a VERY long time, so I only remember it partially, but I believe his name was Mineta (if you know, you know); that one especially. But a lot of them, actually. They may get stronger over time, but we would have to look at them from the perspective of them having not even entered the academy. If they got in and were strong enough to be put into Class A, you'd wonder what the school had to put into Class B. And that's even more of a crime, because we get to know the B-Class later, which is supposedly made up of people with less promising future potential, and they are quite a bit more amazing than people like Mineta.
This leaves a huge stain on the world-building. Sure, it's a story, so you gotta have duds in to balance the playing field - but that's the perspective of the author! The characters IN THIS story shouldn't know they need to please an audience. In other words, the people deciding where to put the students wouldn't need to factor in some balance for anything. They simply want their students' capabilities to be a match, so they can be schooled and trained accordingly.
The same goes for Lumerne. How the fuck did Karl make it in? And if he made it, why are there not many more people who made it? I get that every Instructor has their own means of evaluation, but that alone is a giant fuck up by administration.
You see, in HeroAca, you have a very clear entrance exam and a very clear goal. Everybody has to do the same thing, and everybody gets a very clear rating. It's pretty transparent. Sure, the MC of HeroAca gets in by getting somewhat shady extra points, but even those are clearly explained, and it's kinda obvious that there was a catch with the biggest enemy giving the smallest amounts of points. Anyway, Lumerne simply has different people in every region, and they get to choose what they ask of their prospects and how they fail or pass the exam. That's intransparent as fuck, and it also robs the country of possible heroes in the future. While, yes, you could say that the people who didn't pass weren't good enough, but they probably would have been good enough had they lived in a different region and participated there.
You either want to weed out all the shit in every region or you accept that kids who aren't yet perfect might make good heroes (in other words: People fighting for mankind) in the future, whether you like it or not. Letting it happen that one region might pass on a great future hero, because of the whims of one instructor, while letting someone like Karl in. Again, I like him, but he really doesn't belong there; even if he got better by now, he's a literal joke compared to other characters, and I mean it, because we all know he's there to really just be a running gag, even if he might become a good supporter one day.
It also ties into the beginning of my rant, actually. Because as far as I remember, all asshole characters that didn't make it in the exam with the MC for Lumerne were males. Because a bitchy female character would have ended up falling in love with the MC, probably as a tsundere, and would have been strung along or at least sent out with the thought of entering the next year for sure. The dude who had to forfeit in order to survive being eaten by a monster was never to be seen again. Maybe he will show up again, but I don't like his chances - with this, I mean that he will likely be a foe if he were ever to return, unable to overcome the humiliation dealt by the MC and Lumerne as a whole. But I'm open to having my expectations subverted (for the first time actually subverted, not absolutely catered to instead).
Again, harem-focused. Only female characters get real background, even when it's just crammed into 5-minute info-dumping sessions. Only female characters have extended plot armor. While all the short-term bimbos he dragged around got motivations and pseudo deep talk, the characters that seem to have real issues with the MC at school, of which only one is female (the one from the summoning course), that are recurring characters (because, duh, they go to Lumerne too), have literally nothing to them. We learn nothing of substance about them at all. KARL is part of the MAIN cast, for fucks sake, and we know NOTHING about this bro. We only know his antics because he's of the motherfucking MAIN CAST and appears in the background all the fucking time.
Think about it for half a second, and I'm sure you'll know what I mean.