Shitappa Elf wa Haraguro Ousama no Omocha

Long ago, demons and humans established a peace treaty and vowed to live in harmony. When Fouchen, ruler of the human kingdom Zenobi, arrives at an intertribal exchange, he finds a rookie demon soldier named Walt absentmindedly gazing at butterflies... The soldier's distinctly un-demonic nature fascinates Fouchen, who decides to bring Walt to his castle! But this king has always gotten everything he ever wanted—will his newest plaything meet the same fate as all the others?

(Source: Yen Press)