Ningen dakedo Maougun Shitennou ni Sodaterareta Ore wa, Maou no Musume ni Aisare Shihai Zokusei no Kennou wo Ataeraremashita.: The Guardian of Princess

Leon was abandoned to the world of demons right after he was born because of his lack of magical power. However, his fate changes completely when the Four Heavenly Kings of the Demon King's Army picks him up. The Four Heavenly Kings, the strongest and most powerful figures in the world, had trained him personally and took good care of him. As time passed, the boy became a magnificent soldier of the Demon King's army with the power to "rule" every kind of magic. Leon has been given a new mission to protect Alicia, who will enroll in a magic academy on the island called "Paradise Island." (Source: Shousetsuka ni Narou, translated)