Well I think that

We know from his information that he was a popular boy before, but one day he just became lonely and did not want to talk to people about his feelings.

With this I already have enough to know how it would be related to Osana

Osana in her confession to senpai mentioned that some boys invited her out, but she always rejected them because her heart loved a person, and that person is senpai

Kyuji maybe one day decided to confess to Osana and she just rejected him because she loves Taro, to be rejected he felt so broken that he did not have as much courage as before, so he decided to get away from people to have their own space .

Even so, he still loves Osana and one could say that he is so devastated that he is very jealous of senpai, since osana is in love with senpai and not with him.

In fact this can be thought of as he always goes to the place where Osana and Senpai meet, and looks at him with a sad face or rather disappointment that he can not be senpai