
Jaehyun helps Kang Jun during the time he is being bullied, which allows him to survive at the school. He's also who Kang Jun decides to force himself into liking instead of Yohanz although it doesn't work because he can't bring himself to. Jaehyun is the person who enlightens Kang Jun to the entire situation on Yohan (and Junwoo too) and himself and how they are all viewed by their peers and why what's happening is happening. He's also the only person besides Kang Jun to see Yohan for what he really is. After high school he also fades, though their last meeting is the catalyst for Kang Jun's last huge explosion with Yohan.

They're Korean novels just in Korean! There hasn't been any finished English translation published. RIDI is if you want to try and read the full novel in Korean, and MrBlue is where you need to go if you want to read the manhwa in Korean since it's ahead, but they're all paid if you're trying to support the author and read there ㅠ

Yohan took the photo, and Jun finds out because he has a sneaking suspicion it's him once he realizes theh are next door neighbors. So one day he finds Yohan sleeping in class and registers his fingerprint into his phone. They get close and start going to each other's house more often, and one day while Yohan is in the shower his father calls him and Jun picks up the call. During the call he finds out the true reality of Yohan's relationship with his father, which shocks him a bit. But then he realizes this is the perfect time to check the photo gallery, so he does. He sees all sorts of random strange pictures of things, but also finds a ton of photos of himself since before him and Yohan were friends, back in first year, all sneakily taken. He's shaking when he realizes what it means, and when he scrolls further, he finds the picture he was looking for- the one from the window at dawn, and that's when he knows.
if you want any spoilers of the plot/characters from the novel, lmk