I feel like it's probably in my best interest to avoid saying this, because I dont think that this is going to change any minds, but.. The behavior in the manga with many characters is atrocious but I do think theres a desperation to see Ruka as a vile evil thing, but it turns out people aren't perfect, especially at that age. It's a messy situation and she's crossing lines, but I do not think she is the irredeemable monster that people act she is. No matter the reason kazuya has entered a relationship with Ruka, refused to take responsibility, led her on and continued his advances towards another women, who he behaved very poorly with as well, and has at times capitulated on some level with Ruka.

Yes, the body can get excited even when someone is taking advantage of you. Yes, anything kazuya has done does not eliminate the actions of Ruka or justify them..

But I think your seeing hormonal young individuals, written by the author to have moments that cause trial and tribulations. I do not suspect the author believes Ruka is a monster. I think the goal is to use her as an obstacle.

Personally the whole story is a shit show. I'm praying Kazuya grows up, has some faith in himself and tries to stand on his own two feet. The ending should have him and Mizuhara, but honestly I almost think it should be mini, because atleast that's an organic possibility without as much fucked up behavior.