High School DxD Hero Episode 0: Holiness Behind the Gym

Holiness Behind the Gym serves as episode zero of High School DxD Hero, the fourth season of the franchise. It revisits the story told in the final four episodes of High School DxD BorN, presenting an alternative version of that arc that sticks more closely to the original light novel volume of the same name.

The episode returns to Issei Hyoudou, the perverted high school student reborn as a devil in the service of Rias Gremory and her Occult Research Club. The arc centers on the young devil Diodora Astaroth, who sets his sights on Asia Argento, the gentle former nun and member of Rias's peerage, and on the battle that follows once his true intentions come to light.

By retelling these events in a form that matches the source material, the episode bridges the gap between BorN and Hero, allowing the new season to continue the story faithfully with the series' signature mix of supernatural action, comedy and fan service.