Man, I'm feeling really bad for Chika rn. With all of the other girls that the mc is involved with rn, they all seem to at least see that he's a douchebag and accept that and him to some extent. And I think, they'd also be somewhat open to accepting that he's involved with (or trying to get involved with) the other girls as well, or at least are resigned to it.
Chika, though... She kind of seems like she really sees him as this genuinely nice, if a bit perverted, person. He's pretty chill as a teacher and doesn't judge her just based on looks, helps her out with her family situation and her sister likes him already. Hell, he genuinely seems to be trying to help her best friend who has real people issues and stays there despite the girl basically insulting him all the time. As far as she's concerned, their relationship practically looks like some wholesome route in an SoL VN. So whenever he hangs out with other girls, or Yumi insinuates some things about him, she just trusts and defends him. Which is ironic because Yumi's probably the one he actually legitimately wronged the most among all of them.
The fallout from when Chika finds out is not gonna be pretty. I kinda feel like it'll rival badhomie stuff. Good writing, damn.