@Winterspring Interesting theory, but rather than "inner demon" I felt that the guy inside Jaesin is someone (or something) else from Jaesin that just wants to kill spectres. I'm fairly certain that Jaesin was attracted by the other guy's offer to give him power, which led to Jaesin becoming a vessel. So I think that Jaesin and the "inner demon" are two separate entities. And I actually feel like his markings are different, but similar, to those things. As seen in one of the earliest chapters, he gets the markings from a "fortune teller" and I believe those markings have actually helped him before. (I can't remember exactly when though I think it was when he just moved in?).
TL;DR - "inner demon" =/= actually his inner demon, is a separate entity from Jaesin
markings =/= same markings as those things
I could be entirely wrong though and you could be right, this is just my assumption
Also, red head girl...?