
Spoilers
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he's mc actual brother. In their family only one is supposed to be one sibling to carry on the family legacy and mc was chosen for his looks. In the beginning the brother was chosen because he was the oldest and only child and mc who was younger was sent away outside of the country for a couple of years. If I remember correctly they even gave the entire identity, his name and age, of the first born to mc when that was his brother, that's why the guy was like "you're not the real one" because that's was his identity originally but mc thinks he's talking about the transmigration.
The webtoon is so fast paced compared to the novel.. Seonwoo made his first appearance in the second half but here it was like chapter 30... Only three main points are left of the story so with this space it feels like it will end at chapter 60 MAX