I just watched a movie live action of this manga.In chapter 1 he kissed the kouhai because he tried to help the blonde guy from getting attacked by the kouhai without hurting the kouhai feelings and because he scared that the kouhai would not be able to give up the blonde guy (￣∇￣"). In the movie it happened the same but then the dark hair guy meet the blonde guy and make it clear that he was just lied to protect the blonde guy and there will never be a thing between them and the kouhai said that even if the dark hair guy have a girl/boyfriend, he would never thought of stealing the dark hair guy away. Now this is different than in the manga ╮(￣▽￣)╭.
Then why did the kouhei attack the blonde guy???
In the manga the kouhai still pursue the blonde guy but in the movie, the kouhai already gave up the blonde guy.
This third volume confused the crap out of me many times. I even tried to go back and re-read parts but I still didn't understand half of it. The side story was even a little confusing.