My guess is that it’s what the uke told senpai when he broke up with him, maybe like “I love you, but I’m sorry, I feel more comfortable with Yajima” or something
Wait, I thought the senpai was going to say "unrequited love" regarding Yajima and Kaoru, and the senpai noticed that Kaoru was getting along with Yajima, because somewhere in that chapter or the previous chapter, you could see the senpai notice them. So the senpai thought Kaoru had feelings for Yajima, but it was unrequited, so Kaoru blocked his mouth 'cause he didn't want Yajima to hear that.
Maybe he thought kaoru likes yajima one sidedly and kaoru was getting advice from senpai. Cause yajima did say sumthing about getting advice from ur ex not being cool.
Sooo i reread this and still bothers me.. Did Kaoru really date Senpai?...cause in Ch5 Page 17 i think...he was saying something about unrequited love and Kaoru covers his mouth...i may be thinking too much about it BUTTT its kinda shady lol why did the Author include that scene?? Makes me feel like iT was planned out to get Yajima...but at the same time it seems fine cause he did seems like he did date senpai.......Ughh the scene just bothers me Lol