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Donghyun, taejun's friend who's been in love to him since their highschool days took a picture of taejun and seheon's father kissing and submit it to the school admins the day they are supposed to run away. That's why he chose to took his life because he said he wanted to end it when he's still happy.
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Actually I'm also a bit confused now but if I remember it correctly at the last chapters, Donghyun was crying hard to Taejun's uncle while confessing that he's the one in charge why all of this happens. Somehow it means that Taejun personally know about it. Its weird that Seheon's father suddenly called it off without any reason, considering that he loves him enough to leave his wife and son. I'm sure that they were supposed to run away together until the picture was send.
Its been a very long time since I read a webtoon where I can deeply attached to. I like the stories and development so much. Nothing is force and every little detail compliments to the story. Tho I felt really bad for the dad and the hyung love story, its still true that he destroy his family because of cheating and the MC's will never met without this tragic past. I hope there's a sequel in their college life JUST BECAUSE I DONT WANT THIS WEBTOON TO END.