I think the last half dozen chapters have greatly soured me on the character of Kase, beyond the other problems with her.
It was a grievous betrayal of trust to agree to that bet and Yamada's tearful breakdown in response should have elicited some sort of realization, but it doesn't appear like it did. Asking somebody to believe in you, after you're essentially gambling your relationship on a coin toss is heartlessly inconsiderate.
I can also for the life of me not understand her decision to refuse clearing up the situation to Fukami, unless she won. Neither outcome makes sense, both are emotionally damaging. Either Fukami is left in the dark about the reasons for Kase leaving, or she's left knowing exactly how much damage she's caused when it's too late.
I really hope it's not just so the author can use the uncertainty of her knowledge to force more drama, because the manga is already overusing her as an ankle-high fence between the main characters. Fukami appears to exist just to suffer for the sake of tension and it's becoming frustrating.