From My Anime List's description of the series
In normal RPGs, a hero defeats the Satan king and the story ends. But what does the hero do after that? Ohtorizawa Akatsuki defeats the Satan king in the fantasy world of Alleyzard and returns to the real world. He is sheltered by the international organization BABEL, which protects and controls people transferred from the outer worlds. BABEL is astonished to know Akatsuki has really defeated Satan and brings Satan's daughter Miu from Alleyzard. In the academy of BABEL, Akatsuki confronts other "returners", who have psychic powers.