The author is more interested in you thinking his oc is rad as hell than actually doing any character writing or worldbuilding. At times he is presented as a benevolent ruler who only did bad things in response to the atrocities the other races committed against demonfolk. At other times, he is presented as a self-obsessed snarker whose actions are solely motivated by what interests him at that immediate moment. At other times, he is a flat out sociopath who tortures people for extended periods of time all while gloating at them.
Arno is not a character, he's a stand-in for whatever the author thinks would most seem cool in the moment. Unfortunately, he's also the only character in the series. A variety of waifus, a rival, his children, and former enemies are all introduced yet collectively have less text boxes put together than Arno does alone. Supposedly Arno feels some connection to the girls as at one point he says "Maybe giving love a try wouldn't be so bad", and then nothing ever comes of that. The girls follow him around like lost puppies and gush over how cool he is for solving their problems instantly and how charming he is for giving them literally one compliment.
He's also just the strongest and bestest at literally everything. We never see anyone superior to him in any field, even if it's their specialty and he just learned it exists ten minutes ago. Even when fighting a fusion of two of the strongest demonfolk ever, one of whom is his child and of the highest tier of demonfolk, and the literal god of time, he dogwalks the encounter all while being condescendingly smug while doing it. Yet somehow he was incapable of doing anything more to fix the world 2000 years ago than killing himself and telling the Heroes to fix it.
He's also supposedly heavily invested in exploring this new peaceful world and why no one re the real him, except he never does anything to forward that. After freeing his son(?) from his mind control fusion, he tells his son, "Hey you look into this, I'mma keep going to school."
And then he does. He never explores the rest of the world or even asks the briefest of questions about it. He just goes to school and does school things. He goes to classes and participates in tests, none of which challenge him or teach him anything, he just does school for some unclear reason that I can only chalk up to Japan's general obsession with schools as a setting.
The series fails as a power fantasy, fails as a romance, and fails as a mystery. Frankly this series would probably be more fun if you treated it as a gag manga where the schtick is he's the strongest in the world -- though that too has been done better by others.
Also the author died so don't expect any conclusion or closure. The series suddenly ends mid-duel with no further information. Don't worry, Arno was winning anyways.
