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After reading both the way Jooin ended with Yahwi and Cain all I can say is to me it's clear as day that who is the end game and it's not Cain lol.
With Yahwi it was actually Yahwi who spoke for Jooin and ended things with him and let's not forget the way Jooin was crying and there were no explicit Good byes. To me as a reader while reading it didn't feel like that was gonna be their ultimate meet up and in fact felt there is a room for development in the future.
However with Cain well the bro knew that Jooin NEVER LOVED HIM and whatever Jooin told him were more like he appreciated him and treasured him. And loving and treasuring someone is not one and the same(as said by Jooin himself). Ultimately when when Jooin removed the leash and frees him...Cain goes like he wouldn't look back now and be happy like Jooin asked. So yeah lol it's pretty obvious that where this is gonna go after the time skip.