Kaori’s rule restricting contact everywhere but the school roof never seemed fair to me, and the fact that Yuuki went along with it didn’t make it any fairer. She says it’s “her decision” to pretend she and Yuuki don’t know each other, but pretending you don”t know someone when you do is rude any way you look at it.
Of course, one way you could see Yuuki’s tolerance of this rule is that he simply likes being her only friend. This is backed up both when he got irritated when she talked about nothing but Kiryu (leading to their first fight) and when he gets jealous when a new girl, Yamagishu Saki, breaks all the rules and coaxes a new friendship out of Kaori with her floaty, lazy charm.
When Saki arranges an after-school friday date with Kaori, Yuuki again exhibits his possessiveness, essentially stalking them with Kiryu around so it appears less creepy than even Kiryu himself agrees is quite creepy already. I know Yuuki means well, and under these specific circumstances he can’t very well barge in on the date, but it’s behavior I hope he doesn’t repeat in the future.
Saki turns out to be a good thing for Yuuki, as she handles the “Monday transition” totally differently than he’s been doing. She’s persistent and is able to get Kaori to remember details about their date. She’s even able to put Kaori so much at ease, she starts talking out loud to Saki and others in class; huge progress. Kaori even spots Yuuki, and recognizing him, gives him a look, perhaps cognizant he was her first one-week friend, and a big reason why she now has two.