Maou Ojisan to Keiyaku Seikatsu Hajimemashita

"I want you to become my familiar and protect me." ".....What?" One day, a middle-aged man who claimed to be the Demon King suddenly appeared in front of Rian, a high school girl. The old man even asked her to become his "familiar"! Despite being the Demon King, the old man is surprisingly serious and kind!? And so, her life under contract with Uncle Demon King begins.... source