All the girls have had development, regardless of where they started and currently are.
Now the og ex went in the weird direction and she’s had no panel time in forever.
Ruka is basically Nino from 5toubun, without being a main heroine. Judge her however you like, but she’s been on a long term redemption route since the blackmail (like Nino drugging Fuu and been doing the same since).
Sumi’s is obvious as we’re literally going through it now. She would have more of a chance if she appeared earlier in the story.
The entire manga is to force MC with Mizu together, so there’s that. Her development is willing to accept him from rental to real. Although the fleshing of her personality has shown she’s a piece of shit. Calling back to 5toubun I guess she’s similar to Ichika in a small way. Starting off generic and turning heel. Although not nearly as interesting a character as I guess the author doesn’t want her to go too far down the shithole because she’s endgirl.
The MC is the one who hasn’t developed since the beginning and maybe the self awareness just now will lead to that and thus end this terrible manga.