Junichi used: "Confession" on Kana-senpai. Kana-senpai counter-acted, using: "You can be my watchdog" proposal. Junichi-kun accepted counter-proposal almost immediately. Kana-senpai is pleased that "watchdog" strategy keeps would-be suitors away. Yui-chan (Kana's classmate & friend) facepalmed (mentally), all the while reminding Kana that Junichi's still a (cough-cough) healthy young male. One day, Nature and Vanilla Gods said together: "We're done putting up with this 'watchdog' crap ... Let There Be A Hook-Up!" ... and suffice it to say, there was a Hook-Up, and despite her initial intent to avoid such a thing, Kana-senpai seemed to like it (clears throat) very much. Yui-chan senses "something has changed in Kana's and Junichi's relationship". Kana confirms Yui's suspicion ... irrefutably.
Well hey, why would he turn down such an offer? He wanted a relationship with her, but if that's not immediately possible, he can just work his way up from 'watchdog', it's a position where he has an excuse to spend more time with her after all!
Ah, so her friend turned out to be quite the wingman, reminding her that he is indeed male and has a penis down there. And hey, he also seized the initiative in an opportune situation, knowing it was his chance to push and finally rank up from 'watchdog' to 'lover'. Her reaction at the end was also really adorable.