ok here's my theory on Iruma's origin

He is the result of Merize's blood being transformed by Delkira with his magic. This would explain how Iruma resembles her so much (my AmerixIruma ship would sink but just look at their eyes and mannerisms, it looks so alike!)

I think the blood in that handkerchief before was used for experiments and Delkira and Sullivan had some sort of bet on which side (human or devil) will win in the end - that's why Iruma's parents are so negligent/abusive(?) towards him because this is Delkira's test to get that demon out of him - they're probably from magic also or just humans who had a deal with Delkira.

Sullivan took Iruma because of this bet since he won and kinda also as a sort of remembrance of Delkira. He didn't know that Ali was in the ring because he didn't think that Delkira was waiting there(maybe sealed - but I bet the ring was given by Delkira too before he was gone), as a measure just in case Iruma was a human in the end of that bet - and he's coming out slowly because Iruma's becoming more of a demon by each time

Delkira's disappearance is probably because he thought the bet was more fun and/or this was a way to fix that netherworld-human world rift