
SPOILER
When Seyin was in elementary school, he first met his half-brother, Gon Hanyeol, when Gon Hanyeol's mother went to ask Lee Juho, Seyin's father, to provide Gon Hanyeol with everything he needed. That was the first time Seyin's mother found out that Lee Suho had cheated on her. An agreement was made with Gon Hanyeol's mother, but Seyin's mother went away. Lee Yesin and Gon Hanyeol were born one day apart, with Yesin being born on February 1st and Gon Hanyeol on December 31st.
After that, the two grew up together, studying at the same school and spending time together whenever they could, secretly, since they weren't allowed to go to each other's houses. The fact that Gon Hanyeol was Lee Yesin's half-brother was an open secret, everyone knew it since it had been in the news and then silenced by the Lee Corporation, but still there were always people speaking badly of Gon Hanyeol and his mother.
Lee Yesin always defended Gon Hanyeol, and Gon Hanyeol always supported Lee Yesin, but when they turned 19 everything got worse, since on one occasion, when they were spending time secretly in Lee Yesin's room, they heard an argument between Yesin's mother, who had returned after 7 years and Lee Juho, in which she claimed that how was it possible that the children had been switched at birth.
It turned out that Gon Hanyeol was Lee Yesin's mom real son, and they had been switched because Gon Hanyeol had been born with some problems and they didn't think he would make it. The ones that came up with this plan were Lee Juho and Lee Yesin's grandfather. After learning this everything things got uncomfortable between Lee Yesin and Gon Hanyeol, with them avoiding each other and all that and guilt eating Lee Yesin away.
Lee Yesin felt so guilty for taking Gon Hanyeol's place by right that he confronted his grandfather and father but seeing that they couldn't do anything against them he come up with a plan to make his grandfather recognize Gon Hanyeol, he would live the wildlife with the money they had given him to keep shut and ruin his reputation so he didn't have any other choice but put him aside and recognize Gon Hanyeol, he also contacted lawyers and reporters that could cover the news and expose what they had done, but the day before the gala in which he would expose everything he got a call from Gon Hanyeol telling him that their grandfather had called telling him that he would recognize him, after that Lee Yesin called him and his grandfather told him to stop all his plans and to come to the gala with Gon Hanyeol and they went to buy a suit for Gon Hanyeol together.
The next day they showed up together to the gala but as soon as they were inside they heard people wispering bad things about Gon Hanyeol, and blaming all the bad things that Lee Yesin had done in the last few months on him, Gon Hanyeol couldn't stand this so he left inmediatly and when Lee Yesin tried to go after him he was stopped by someone saying that his grandfather wanted to speak to him, it turned out that everything had been his grandfather's plan to make Gon Hanyeol look even worse and to threaten Lee Yesin to stop. At this point Lee Yesin understood that there was nothing he could do against them and he was even grateful that he didn't have to be the one being attacked and getting all those bad looks, so he agreed with the condition for them to leave them alone to which they agreed, after that he left the party after staying for a little bit and picked up some blue flowers he had ordered for Gon Hanyeol and went to Gon Hanyeol's house only to find he had commited suicide
I need someone explain why mc is the way he is?