
oh my goodness i was so caught up in the happiness and cuteness that i completely forgot about the sakuras ! you brought up a rlly interesting point omg ! i agree honestly, why else would he know about sakuras? but he also could have just gone into the human world(like sullivan did) and seen them. OR maybe he fell in love and had a child with someone but had to go back(or maybe still there, das why he missing), i think those theories are interesting and plausible !

I feel the same way and that's another good possible theory!
He could have gone to the human world and had a child (possibly Iruma) and confessed such to Sullivan before dying / or whatever it is that happened to him? As such, Sullivan decided to look for Iruma and raise him as the next demon king since he's the actual heir??? Ahhh I love this theory but it seems we'll have to wait a long time and see if anything matches thank you for this ٩(๑❛ᴗ❛๑)۶

yes yes that would be a very plausible reason for sullivan to go ! because also why would he do something so risky when he COULD just adopt a demon grandson, why go through the trouble for iruma of all people too ? i don’t mind waiting because i love this series so much but i hope we find out soon !!(≧∀≦)

The fact that he was changed in a wicked phase might mean two things though: the demon king, after acquiring power somehow (if it's the case) he was able to change his appearance to that of a demon or since he's an actual demon / half-demon his appearance is that of a demon.
In Iruma's case, if he was a descendant, being it from a half-demon (again, being it the case) he would only be a quarter-demon reason why he's able to wield magic but his appearance and three-quarters of him would be human (which we have seen are special beings as they are believed to be legends in the underworld).
Another reason could be, as seen in other works, that half-demons are more powerful than their peers (demons or humans) because of the mix of their blood. Iruma is powerful as a human as well (it might be exaggerated but he has great qualities as human).
That's what I could add as theories (●'◡'●)ノ

At first it might have been portrayed as "he got lucky" but we've seen how much he's grown and how powerful he's actually gotten by himself. Even with his friends' help, him on his own has his own way to handle things and reach out for his own ambitions. I honestly think he's as powerful as any demon and he proves it every time (no wonder Azz is so captivated by him (▰˘◡˘▰))
Someone below made a theory which hinted at Iruma being a reincarnation of the previous demon king, either that of from his lineage, so I just came to add (even if it's obvious) that the previous demon king was human or at least half-human (since the legendary leaf is a sakura tree which is human). It's interesting considering they're the strongest and yet somehow they're humans who are 'legends' in the demon world (only high-ranked demons know of their existence, like Iruma's grandpa).
So now I've got many questions regarding the demon king itself: what happened to him? Was the one Iruma saw a simple illusion or...? If the previous demon king was indeed human or half-human (as our lifespans are short) was he somehow sealed so he wouldn't exactly die? Is Iruma then his reincarnation or has any of his lineage in him? Is that why Sullivan sought him out, maybe as a promise to the previous demon king? Sullivan definitely is hiding something big (I don't doubt his love for Iruma though).
Anyone is welcome to add ヾ(❀╹◡╹)ノ~