I state my theory here again haha (づ￣ ³￣)づ
My theory is that Iruma is related to the demon king in anyway you can think of, either he is the son or the very reincarnation, but I am sure that he is connected to him one way or another, because why would Sulivan specifically kidnap him, Additionally, Iruma seems to have been trained since childhood to escape even the worst situations, and having lived a much harsher lifestyle has made him highly skilled at adapting to the demon world. Think about it, it was too coincidental and very much intentional.
IKRRR and to add to this everyone keeps getting this familiar feeling of delkira from him be it during music festival or further arcs. And as he is gaining confidence and showing is personality the vibe is similar to delkira. I also think ali san got something to do with delkira too. And also he fits the future demon king description exactly
OOOOOH it's a very cool theory
Two things one ; he his a goooooooooaaaaatttt. Second if a human can't stay long in the démon world since his body will become weak how does iruma has no negatives effect ?