I tried to pick two characters each!

I fucking LOVE Shizuku. She is SO GOOD. I've been a huge Shizuku fan ever since I read Color of Myself AGES ago... she is just so earnest and genuine and the core themes of her story being about "staying true to yourself" makes me very happy. I find it criminal how little people talk about her story, like, at all! This is so good and it makes me feel like I'm living in the wrong world lol. To go out on TV, she handled her "reuniting" with Arisa and Cheerful * Days so well. I feel so excited whenever a Shizuku event is announced because I know it has a very real chance of being one of the best in the game.
I deliberated for a bit on who to talk about here (I wrote this character last out of all 4) but I think I'll give it to Rui just because I really wanna talk about him lol. I really enjoy the direction of someone who is uncompromising in their ideals and was isolated by people finding him weird. Rui finding people who will accept him and love him for who he is just means so much to me. He is able to put his full passion into things and people will be right beside him the whole time. It's a good story about acceptance and treating others who are different from you in a positive way. I find his little arcs like the Meiko-Rui side story trilogy or stuff like Kamisports-Kamikou with him making friends and growing accepted into school so simple but beautiful.

I wouldn't say "extremely" personally, but I do think Haruka is sadly one of the lesser developed. I felt like for a long while they didn't really have as much concrete ideas on where to "move her character forward." Some of her better development came from events that aren't even her own (ex: Emu's sports mix is weirdly way better as a Haruka event lol. This kinda only furthers how dire Emu's mixed situation is...)
But, I thought Painful Hope was a pretty good event for moving her in a good direction, and the latest event, Lead to Shine, was very good imo. I think they've finally figured out a role for Haruka in the story and I hope they pounce on this some more by making her role as a producer relevant. It would be kindof insane if they didn't, but I feel like they sometimes fail to utilize some of MMJ's themes well, such as Retie having Ayumi go completely MIA for an entire calendar year with not even a throwaway line or a side story about Airi retying with her.
Another character I'd call on the weaker side is Kohane. I really don't agree with people who clamor for a backstory for her because that would defeat the purpose of Kohane being a character who had nothing for her finding a passion and joy. But I don't think her development has appealed to me as much. She has some great moments (Kohane's solo song, Traits, is really fucking good), but something about her overall growth just... hasn't really clicked with me...? I think it's due to On Your Feet personally, which I felt really disappointed by. IMO OYF didn't move her character in an interesting direction. ORS helps it a bit but it didn't salvage it for me. Her dynamic with An and their growth together as a duo has been superb though, with Kohane feeling proud that she's someone An believes is worthy.

I'd like to add I still really enjoy both of these characters. They are by absolutely zero means "bad" and there is stuff to love about both of them. I love the story in this game and I'm always excited to see what's next. I don't hate anyone at all. I think they're just characters where the cracks have fallen through a bit more. Haruka with some lacking early writing, and Kohane a bit more recent writing. (albeit not like her most recent stuff lol)
This was a pretty cool ask, thank you!! Gave me much to think about and voice some thoughts.