@Constellation of souls
Spoiler!
Maybe it's a stretch for me to associate it with the plot. As for why they had s3x, it's necessary to cure Wi Soyeon's disease or else she'd die. It's just that after they did the deed it felt like he's becoming more human and slowly but surely can identify what kind of emotion he's feeling unlike before that he'll just brush it off if he doesn't understand that emotion. And if I'm not mistaken, that's the trigger for Cheonma to see blue spirit as a woman. He's still ruthless and cruel tho
I don't care about your taste!