Responses
I think in their past lives, Jooin was Cain’s master I forgot the name of the time period but think potn time period
If we're looking at it in a realistic point of view, Cain doesn't speak perfect Korean and Jooin means master in Korean. Or we can later see in the story they were in a last lover/sever master relationship in a past life but were reincarnated as Cain and Jooin.
the whole master thing is weird and sus idk if i missed some part that explained it more but i really dont get it.. i still like cain tho so