From what I hear, Seira making a contract with Shinwoo is a pretty popular theory. I also like the idea of Rael making a contract with Shinwoo, because they're in a situation where Seira needs help and Rael alone is too weak, so they have to cooperate and Shinwoo is strong base material as well as loyal to his friends.
SPOILERS (but not really, I just read spanish so I get the chapter 2hrs early. there's nothing big here):
so basically, something really similar to what is about to happen? since rael did just show up, and seira does need help, and this is all immediately following what happened leading me to make my original comment.