Yuusha, Yamemasu

Leo Demonheart defeats Demon Queen Echidna's army and saves humanity, only to learn that the power which made him a hero now makes people fear and exile him. With nowhere to belong, he goes to the enemy he defeated and asks for a job. Echidna refuses, so Leo disguises himself and persuades her four generals to let him help rebuild the army in secret. Instead of glorious battles, he finds exhausted workers, poor training, broken communication, and almost no resources, forcing him to use management skills as often as combat strength. The more he knows the demons, the less simple his old image of the enemy becomes. Leo's real reason for joining them, however, reaches far beyond employment and into a past that changes the meaning of his heroism.