the author is out of ideas already or something ? she keep using the same jealousy trope. it's starting to get annoying and lazy and not interesting anymore. what the hell with you Yamada, how many time Kase told you she only have eyes for you, and even after all that time being together you still don't have any trust in Kase ?
It's not that surprising, and it's rooted in something quite different from when they were in high school. Kase is rooming with an attractive girl that shares her interests. If she were living with Yamada instead I doubt we'd see nearly as much of Yamada's insecurity. Even being told that she's the only one Kase loves, there's always room for worrying. I mean, if your partner were sharing a room with someone you viewed as more suited to them than you, would you never worry? It doesn't help that Kase's roommate treats Yamada rather coldly.
Honestly a lot of the problems here would be made much easier if more people actually knew about their relationship. The only person who knows so far is Mikawa. Fukami thinks that Kase and Yamada are just friends, and that Yamada is being overly clingy. If and when she finds out they're a couple, I'm sure she'll be apologizing to Yamada.
It is frustrating that Yamada still worries so much when we all know she doesn't need to, but we know more than she does about... everything, really.