My Girlfriend's Ex-Boyfriend

I was never curious about my girlfriend's past...until that incident! Now I'm just dying to know! Who is this exboyfriend? Why is he acting so friendly? What happened between them in the past?! A love triangle that isn't so typical. [Winner of the 2nd LEZHIN World Comic Contest] Original Webtoon: Lezhin, Naver Series, Ridibooks, MrBlue, KakaoPage Official English: Lezhin US, Toomics, Tapas