so i do get that theyre not actually brothers. nanhwa is probably some kind of spirit that yeonho met during that accident in the mountains. i assume that, since hes a mountain spirit, hes probably never had much human interaction. and i assume once again that he probably thought being brothers is as close as he can get to yeonho, so thats why hes manipulating reality (?). i could be wrong.
but still, everyone and their mama thinks theyre brothers, so... the author definitely knows what theyre doing...
the comments are frying tf out of me