Honestly, I think that many people are very hard on Kase, even unfair. It almost makes me sad that people see her like that...
In 3 years of relationship she has only made two big "mistakes". Being jealous of someone who gives your girlfriend flowers and calls her "Yui-chan" (put yourself on her shoes) and agreeing to a race to leave the dorms. I didn't like that arc either, but I blame the author rather than Kase, in Japanese culture they literally left her no choice. Social pressure from her classmates, personal debt with Fukami and an authority figure (Inoue) ordering it. Trust me, I worked there for almost two years and the social pressure and relationships with mates are terryfing, almost unbearable for occidentals like us.
In the next chapters she is really sorry, apologizing multiple times, making terrible faces of regret, promising that she'll reflect and giving presents to Yamada trying to ease the pain she has caused to her. She sure is dense, but she loves Yui more than anything.
In the first series and in the first half of this one was Yui (who I adore and worship) who made the mistakes due to lack of communication. They are still very young and they 're growing together. Life sometimes is not like fairy tales.
Anyway, I'll eat my words if she doesn't eat curry with her xD