Is the guy in his past and the guy now not the same?

Or he is the guy but pretending he's not because they swapped place for someeeee reason ┑(￣Д ￣)┍

I think that too

I would be disappointed if Hangyum was not the one he meet in High school. If that happen the webtoon would lose some of the grit and glue from the beginning. The webtoon has provided clues that is the Hangyum he meet in the past. IF not and he is the other one, the twins had to talk about Hajin. That twin would had to know Hajin by sight. Or Hajin was talking to both twins in the past. I personally think Hangyum was behind Hajin getting offered the job as an stylist. Hangyum was the one that took the call and accepted Hajin's terms. "whatever I desire." kind of hints he was behind it and causes me to believe he knew who Hajin is. Just from that and a few pictures and comments I get Hangyum was behind it but that that is not the point of this message.. However, the biggest support of this is when Hangyum was telling Sangjin, Hajin had called, Sangjin did not recall who that was until Hangyum reminded him. Sangjin says "Oh! that male fan of yours? that is odd. I almost thought he wouldn't call back. So he really was a huge fan, huh?" Hangyum says " I told you, didn't I? That he'd call sooner of later." That to me implies he understood Haijin's character. How if that was the "first" time they meet? In chapter 4, we see Hangyum think "a sight for sore eyes." which means "a person or thing that one is extremely pleased or relieved to see." If this was not the one he meet in high school, why would he think that since this would be the second day that meet? Hajin comments to Chunghoon that Hangyum having the same personality as the in high school. Normally, (identical) twins have their own distinctive personalities. their have their own individuality. They would not have the same personality. When Hajin asks Hangyum and he said" You're mistake me for someone else because that is not who I am." In chapter 6, Hajin will add to the moment by thinking, "He was... frowning when he said that." Hajin says," No, I'm talking about Saewoon high school, sophomore year, Hangyum Lee." Hangyum reply was humm off? It kind of seems he worded that carefully. Maybe he is pretending to be Hangyum Lee, and his brother is being him? Maybe His brother was the one in rolled in high school but it was this Hangyum that engaged with Hajin in the past. In Chapter 6, a few pages later, I think when Hangyum says "you and your usual temper" is referring to some one he knew for a while... Not really someone you knew for a few days that has not threw that temper that much. Also, that image Hangyum exhibits nostalgic expression and gesture to me. In chapter 9 Hangyum plays a sing and Hajin thinks "This song." I kind of get the impression it was a song from the past with Hangyum. In Chapter 10, Hangyum knew Hajin hated carrots. Even Hajin is like "He knows that I hate carrots.. yet he's not the Hangyum I know? How does that even make any sense" I think it is funny that Hajin started out to being a stylist for all members of eight nine but in chapter 10, Sangjin says "since you're Hangyum's personal stylist, more or less."
I do believe this is the Hangyum he meet because it is fitting together. When the twin returns and it turns out it was the twin, it would just seem to undo everything the webtoon has already setup. However, I could be reading into too much. What have this Hangyum knowing things he should not and having an interest in Hajin like meeting someone you lose touch that you like/love in the past instead of just meeting some one on sight. Hangyum character kind of seems cold, distant, disinterested but he seems over familiar with Hajin a few times at the beginning and it does not match his put on persona. One would be chapter 2, when Hangyum grabs Hajin and asks for his name.. The etiquette in South Korea, "South Koreans consider it a personal violation to be touched by someone who is not a relative or close friend. Touching, patting, or back slapping is to be avoided during interactions. " Even when Hajin asks him, Hangyum baps him in the face and grabs on to Hajin in public to get close to tell him, "that is not who I am." To me, Known Japanese/Korean cultures, making friends with people from those cultures and dealing with a Korean boyfriend, I found Hangyum grabbing, bapping, and other actions with Hajin, a bit weird if he was not the Hangyum that Hajin knew. However, again I maybe reading into to much and it could be the other twin that knew Hajin.

This is very insightful (≧∀≦)

Thank you. I was trying to help. Sorry for the typos.