Well actually, he didn't. He left him, and at the end he said "hi" showing that he at least FORGAVE him. He didn't end up with him, though. Only in one of the side stories (which is non canon) hope this helped clear up any confusion. (⌒▽⌒)
What??
Sis they did. They end up together. The epilogue follows the main storyline and they lived together
Well actually, it just follows the storyline but it wasn't the actual ending. It was the "what if" you can clearly see that it just follows what would happen if the main storyline turned out differently. The author even says "that's the end of the what if side stories" so them living together is what would've happened but didn't.
There's no "What ifs" written on Yule and Kyungsoo's part. That was the actual and main ending of the story. The two of them ended up together. His brother and the friend of Kyungsoo were just the "what ifs". The author confirmed it herself on her Twitter account. Work on your reading comprehension please.ヾ(❀╹◡╹)ﾉ~
Actually this is my 5th time reading this. I love Yule from the start and I'm glad Kyung-soo really ended up with Yule(▰˘◡˘▰)