One of my theories is that Iruma has to have a demon soul or something, but the way he was brought up by his so called "parents" changed him to be a submissive person. There is also the fact that in this manga, Ali-san's name was Alicred right? And Delkira is the opposite, the l and r might have been interchanged on purpose.
The thing about the ring is also one of my theories. Sullivan didn't feel a thing during the crying of the ring and he is rank 9, so we can assume that the reasons why Iruma didn't feel it is because he was human, he has less sensitive hearing, or beacuse he has a high ranking demon soul inside.
The fact that Delkira in this manga has rings that look like Iruma's ring on both his hands on all his fingers could mean that they have a connection, as Delkira says that Iruma was the "least demon-like demon" emphasis on the demon-like, not "human-like". So what if Delkira was a cambion?? I mean Iruma could be his son then, and Iruma just had all the features of being human, and the prophecy of the next demon king was on spot.
Ali-san is portrayed as a guide right? We already have theories about Ali-san being the demon king, but what if he really is, and that he would only regaing his memories the moment Iruma reaches a certain ranking? I mean Sullivan was trying to brainwash Iruma into not talking about the ring, which means that he knows something. How could Sullivan just have the coincidence of having Iruma, and telling Opera after that Opera would know later?
This are dome of my theories and I'm signing off;
