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
if you want any spoilers of the plot/characters from the novel, lmk