High School DxD Hero Episode 0: Holiness Behind the Gym (Series)

Douglas, once known as a legendary enhancement spellcaster, is now little more than your average, weak old man. On top of that, he was kicked out of the hero party, and eventually had his adventurer's license revoked... And that's how he ended up a vagabond, wandering aimlessly. After an encounter with Rabi, a cursed little girl, a new adventure begins... (Source: Square Enix)