Unlike the first reviewer, if you stick out past the first couple of chapters, you can really see just how much these characters grow and learn from each other. If there's anything else you need to know about this series, just know that everyone - not just the main leads - get the chance to grow up and develop beyond what they're initially seen as.
What's always annoyed me about shoujo/female-targeted series is that the series rarely talks about girls' friendships and how much they can grow from just that. Because each girl is so, so different, at first, and they really don't understand each other. They merely become friends by circumstance and ostracisation from the rest of the school, and the only reason Nari (our main protag) is dragged into this is because of coincidental acts of kindness and an old friend who she never really understood anyway. It makes sense that she doesn't feel like she belongs, especially when she compares herself to these seemingly perfect girls (and who hasn't done that?), but that's the thing - the more she learns about these girls, the less she puts them up on a pedestal, and the more she values herself and grows to value them and their friendship too. Because each girl has their faults - this is always pointed out - and they all grew up differently, having different understandings of their own worth and the world around them. And that's true about most friendships; but in that same vein, the more they understand each other, the more they care for each other.
Nari grows from being self-deprecating, insecure, envious, and a pushover, to someone who can and will stand up for herself, snark back at those who gossip and bully her, and fiercely protects her friends without giving a damn about how pretty they are (unlike how most of the other girls see them). Seonji, shy and sweet and naive, has a lot more complexity than just the running gag of her not knowing things - her story explores how someone, in the face of all of the crap that life gives her, can still have the ability to be gracious enough to leave behind the things that have hurt them instead of being petty and vengeful. Her strength lies in her ability to move on from being hurt and to finding happiness in her friends and herself instead. We're now on Mirae's story, and from what I can tell she has a lot more discomfort and hurt to explore, and we get to see more of what it's like to hide behind an ever-present mask of cheerfulness while always thinking that you don't deserve friends. And as for Yoona, we've also yet to reach her story. But we do see her growing up too, slowly valuing her friends more, although still very much remaining her abrasive self. The relationships with their families are all different too, and the series showcases that. Even the bullies are all different - some change, some don't understand why they'd need to change and continue to be toxic, some become friends, and some just fade away from the girls' lives, just like in real life. The background characters aren't always nice, but neither are they mean - they just act with their own understandings of the situation, which isn't always the same as the main characters, and that's entirely fair. Everyone is very much an individual with their own agency, and it's amazing.
All in all, they're all wonderfully complex characters who all learn about each other. More importantly, they learn how to be good friends to each other, which is something that takes time. This isn't a series that starts off sparkly and happy - it's a series about how different people can be, how others' judgements of us does affect us, how friendships are only truly formed because you come to learn and care about each other, and how people can change. It's wonderfully realistic and fulsome in its depictions of human interaction, and I cannot recommend it enough.