Comment by Grandpa's condom on A Casual Friend - Chapter 70

@Grandpa's condom there is no reincarnation. Dongha was pulled into a novel written by Sungwoo (his first love and the guy he kissed at school, who left after someone filmed them together), Dongha didn't want his sexuality to come out so Sungwoo didn't out him either and accepted his fate, basically. 5 years later Dongha received a novel written by Sungwoo and got hit by a truck that put him in a coma. That's when he was pulled into the novel and met Yujin (who is the novel version of Sungwoo). The thing about Yujin is that his biological name is Yu Sungwu. The end where Yujin gets stabbed, he pushed Dongha back into the REAL WORLD and he awakens from the coma. It's obvious that him and Sungwoo have made things right between them and are dating. The comment about his name being Yujin is basically a petname of sorts his parents called him when he was little. 

Yujin (novel version of Sungwoo and birthname is Yu Sungwoo)
Yu Sungwoo (real life first love of Dongha and whose petname is Yujin)

Hope this all clears it up for you

@Jesse Miller then both novel and real version is different of sang? Ml? Thank a lot for taking time a clear me. But which one mc love me most

Sorry i accident click that one instand of happy

@Skylar Both of them. But I think he still thinks about novel Sungwoo because everything was going alright between the two of them and there was guilt for leaving him behind. Kind of make me think that he feels as if he's leaving real world Sungwoo behind again just like he did when they were in school. But like I said...novel and real world Sungwoo are the same people, so he loves both.

@Jesse Miller like real sungwoo transmitted in the noval sungwoo and that's y both r same?

@Skylar No real Sungwoo is the author of the novel Dongha went in. They have the same names and look. But remember this is a novel so novel Sungwoo aka Yujin and Dongh both have different character backstories and families. But the main point is, by getting close to Yujin, accepting and loving him...he can finally make up for the mistake he made in High School towards Sungwoo. He fell in love with Yujin but since he woke up from his coma, thanks to his journey in the novel, he can't finally have his happy ending with his first love. Aka Sungwoo. Novel Yujin serves as eye opened for Dongha. Making him come to terms with his mistake, accepting it, fixing it and accepting his sexuality most of all. Because that was the reason why he "abandoned" Sungwoo in high school when that video circulated. He loves Yujin because he was stuck in that world but he was scared as to what would happen if he woke up from his coma. Would Sungwoo still be mad at him? You know, so in that sense he'd prefer to stay in the novel. However, when he woke up...he got his happy ending. It's a bit complicated to understand, I know lol.