I am on your side!!
Sometimes we really won't like what others like so you not getting it on the Kirio hype train is fine. There're also aspects to a character you would find irritating or annoying even if they're your favorite and especially if they're not your cup of tea.
Me though, I like Kirio's evil character design. He's not the dumb af canon fodder antagonist there for the MC to face slap. Not the failed by the system, victim of injustice revenge type. Not a hero turned villain disillusioned by reality. Not a villain with savior complex under a misguided ideal. Not the one with a sad backstory to justify his evilness. Kirio is just evil for the sake of evil. He's bad because that's who he is and that's what he thinks he and every devil should be. He's unapologetically a devil true to his nature as they originally were. He's an evil mastermind who plots behind everyone's back, sowing chaos and discord everywhere, and ten steps ahead of the good guys. He's a good antagonist, perfect foil for Iruma who's a direct opposite of him. He makes Iruma shine more and grow as a character because well he's being high-key naughty.
I'm just tired of characters introduced as villains or villainesses only for them to be good. So, Kirio is interesting for me.
Only downside for Kirio is he's a perverted unhinged obsessive freak. Which can be ignored or be desensitized once you got used to it (freaky, obsession, unhinge is prevalent in this manga and some other anime and manga as well, it's nothing new or special we haven't seen before)
don't hate me but I don't get the hype on Kiriwo both on the OG manga and this AU. He just seems like a pampered crazy ass villain to me