Maouoji-san to Keiyakuseikatsu Hajimemashita

One day, the high school girl Rian ran into a strange man who claimed to be the "Demon Lord". And he asked her to become his demonic familiar? Despite some bad demonic habits, it turns out the Demon Lord is actually a pretty nice guy? It's a happy buddy comedy between a demon lord and a high school girl tied together by fate!

(Source: Comic Polaris, translated)

Note: Chapter count includes three volume-bonus and two extras chapters.