The Demon Boy and His Mage Bride

(No System, no transmigration, serious fights, no overpowered hero, one female lead and a romance.) After escaping from a research institute, a boy carrying the heart of the demon Lucifer is defeated by two beautiful girls and forced to become their servant, setting off on a tale of magic and supernatural powers... Some elements of the Nasuverse setting were used for inspiration. It leans toward Western fantasy, and the country's technological level is roughly that of the late nineteenth century.