Gachikoi Maou-sama

A brave hero set out to defeat the Demon Lord for the sake of world peace. When he arrives at the Demon Lord's castle, he is greeted by the Demon Lord herself, waiting for him with a marriage certificate in hand! "Please marry me!" the Demon Lord asked eagerly, which only made the hero more suspicious―? A romantic comedy between a Demon Lord in love and a salty Hero, with the fate of the world at stake!

(Source: Kadokawa, translated)