White hair guy - Minwoo had a school love - tan blue eyed guy - Chuljin. Overtime because of multiple situations he thought that all this time Chuljin was using him, playing with him and also something happened that made them finally break up - we don't know what exactly yet.
Even after break up Minwoo was obsessed with Chuljin and was looking for guys who are similar to him in one way or another. But once he met a guy - Norman, who was completely different and even felt attracted to him. He thought that he was finally free, but then suddenly Chuljin came back and Norman was also distancing himself slowly.
Minwoo was very angry at Chuljin, because he thought that Chuljin came back just to make fun of him again, use him one more time. also he was mad at how stupid he used to be back then and projected it onto Chuljin too.
Then Norman finally left him, he became completely devastated and let Chuljin to come back into his apartment. But when he finally came to feelings he realised that he was still mad at Chuljin and that he wanted revenge. So he decided to do to Chuljin, what he thought Chuljin did to him - to act like he's in love and then break Chuljin's heart, so they'll be even. That's where we're now and as we see it's kinda hard for him to act that way as he still has feelings and it shows

Can someone explain the third couple’s relationship to me like I’m five?