MC's ex lover Jahea (the blondy guy from another group) was jealous of him and used his feelings to make him miserable. He told MC to ask money from a loan shark and MC has to wh*** himself to pay his debt because his sister was hospitalized.
MC trusted his lover but was betrayed/blackmailed and used.
Jahea never loved Shiyeon back but he likes to have his affection. He has a complex of inferiority and MC is more like a trophy.
Now that Shiyeon is ghosting him, he's pis*** but it's refreshing to see that he has to make a move to have his attention when before he hasd MC in the palm of his hand.