I haven't read the manga so can't compare them, but I especially like Koyun's character design. There's something very appealing about the contours of her face and the shape of her eyes - she has a "wise beyond her years" look about her.
And yeah - it's probably true that Koyun and Minato are similar, but in opposite directions. Minato is sort of an even worse version of the kind of person Suzuki in You and I Are Polar Opposites wanted to stop being. It doesn't appear that he's consciously manipulative - rather, it's that, in Suzuki's words, he constantly "reads the room" and tailors what he says and what he does to what he thinks will fit him in best.
But conscious or not, that still works out as manipulative, and Koyun sees it and isn't having it.
But Koyun goes too far in the opposite direction. First off, she fails to correctly read the room - she's hypersensitive to being teased and more to the point, she misunderstands it - the examples of her being teased in elementary school that bother her so much are relatively low key and appear to be, and are specifically said to be by other characters, examples of boys in particular saying teasing things because they like her and that embarrasses them and that's what boys do when they're embarrassed. But she thinks they're all just mean-spirited and deliberately hurtful, so she shuts them out entirely.
And Miki is actually sort of a mix of the two - shifting back and forth between her public persona, who constantly reads the room and plays the role demanded of her, and her private persona, who hides behind her wall and couldn't care less.
And then there's Youta... He appears to be entirely the opposite of the rest - he's entirely and completely open, with no walls and no poses.
Unless that's another type of pose.....
An additional note - I just read through the first two chapters of Class de 2-banme and yeah - the adaptation's the problem.
For instance, there's a devastatingly vulnerable look of sort of hesitant and shy sincerity that Asanagi gets when she's being completely open with Maki - and I don't recall even the faintest hint of that in the anime. The adaptation can't have included that expression - at least not that well rendered - because I have no doubt I would've noticed if it had. And if they had just captured that one expression and shown it in its full glory a few times (most notably when she showed him her list of hobbies and interests, said that she was "stupidly honest like him" then asked him to be her friend), that would've gone a long way toward establishing the sincerity that I didn't see.
But I'm going to let it go at that. Tempted though I was, I didn't even go on to the next chapter to see how the confession played out. Giving the anime a chance first.