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This story deals with such heavy issues about society and conforming to the norm.
Saegye is bearing such a heavy burden. If he doesn't help, someone innocent would be framed but if he does, he exposes himself and the rest like him to a cruel and unforgiving world. Either way, he bears the guilt and I think its unfair for such a young man to bear so much guilt and hurt for something he's not at fault for.
I don't think he could help by just mentioning that he saw the culprit push the idol down the stairs. If it was that easy, Han could have asked someone else to do it for him, it didn't necessarily need to be Saegye.
Think about it, if Saegye was the one to say it, people would question why was he there at the scene of the crime, how could he prove he was there, wasn't he student supposed to be in school? So that definitely wouldn't work, since he is the only on who could lie, the brother can't be asked to cover for him.
But i think whoever Doyoung chooses as the real hyung becomes the real hyung, whoever he thinks is fake, becomes fake.