Ikr?!! THat's the million-dollar question that pops up in every person's head at the end! I wish we knew..
I am certain that it was Dai because of what Jahee felt around him, how the man spoke of Dai's taughts and the fact that the man asked to hold him.
I think the guy in the cabin was Dai .but the reason why he denied it and made jaehee went to America to find him was the last word he told to his father before forced to go to America "ill never come back here as long as i live"
Damn this story is full of extreme emotions, but I guess that's what makes it good. You just keep on reading without wanting to stop.
I do wonder though, that guy at the end, was that actually Dai? Or was it just another person who crossed paths with him? He was just so vague ><