Spoiler.
They met in highschool when Yeonwu was an exchange student in America. They became close because they were the only koreans in school.
Thank you (≧▽≦) Another question. When he discovers his brother's relationship with Dojin they already have something and hide it, or is it something that is yet to be developed?
Spoilerrrr again (≧∀≦)
Actually when they met yet again in the university, Taehwan told Yeonwu to pretend they don't know each other because Taehwan was still mad at Yeonwu when Yeonwu tried to kiss him (while he was sleeping at their usual meet up spot) in highschool. So when Taehwan found out about TaeheexDojin, they dont have a thing yet, but Yeonwu is still in love with him.
I'm waiting for the story of Taehwan with Dojin's friend because I see great potential ⊙.☉ I suspect that they studied together (and that's why the friend knows so much about him), OR they have something that Taehwan himself doesn't admit (maybe the place where he disappears?)