Maougun Saikyou no Ore, Konkatsushite Bishoujo Yuusha wo Yome ni Morau

"Use thy magic to render the hero powerless." Lido, the strongest of the Four Heavenly Kings, has infiltrated the human realm to carry out the Demon Lord's order. He was trying to gather information on the heroes, when he somehow ended up at a marriage agency. He is introduced to a beautiful girl named Rena, who is also a hero, and it turned out Lido and Rena were each other's first loves when they were children. Unaware of their past love, Lido and Rena get married, ostensibly for mutual benefits only, and they were supposed to live separate lives together....... Their marriage is a sham to deceive the world, with maximum fighting power but zero love power! (Source: HJ Bunko, translated)