Welcome to Demon School! Iruma-kun: The Demon's Self-Cultivation

As the last demon of the Sanguine clan, Olivia swears to make the clan thrive. A deathbed wish from the clan elder, the family's gradual decline... None of that exists! Living alone in a huge castle with more money than she could ever spend is simply Devi heaven! Wandering everywhere to punish evil and uphold good is what she ought to be doing, but where did this ridiculously good demon come from, stealing her thunder again and again?!