>>182156269>this crazy bitch won't leave if you won't go out with her, you should go out with her, she is your responsibility because I say so.
Ruka says she is in love with him, so what? Why he needs to succumb to her will? Because she threw a temper tantrum? Not really the whole thing happens because she knows Chizuru is a rental girlfriend and Mizuhara is afraid she will tell someone and her secret is out. Sure Kazuya's lies will be exposed as well, but I'm pretty sure in his case repercussions won't be too severe. He does what he does mainly to protect Chizuru and his grandma.
Honestly we don't even know if Ruka would ever reveal her secret, it was Kazuya's idea to have her vow( which she nearly disregard a couple of chapters later when she learns how much Chizuru is involved in his life, sure they can absolutely trust her)