Mc or yuri is actually a victim of dark magic, forbidden magic, the perpetrator has not appeared yet. His soul was accidentally pulled because he was like on the verge of death, his soul is currently divided half in Korea (coma) half in his current world. So when he changed, yuri felt pain because his soul was like a counter, later in the end he was forced to let yuri go back because his soul was damaged for too long being divided so he left his soul intact first after that michael picked his up again in Korea and he brought yuri back.

Later there was a scene where yuri was kidnapped by the one who used dark magic, yuri was tortured, it was quite scary, I read it in the novel, I don't know if it will be in the manhwa later. 

But in the end they ML will marry Yuri and both the end of side story and chapter will have a intenses and hot scene of their s e x.