The Highschool DxD light novels by author Ichiei Ishibumi are loved by fans. So much so that one guy wrote a 55-page long thesis (via Reddit) on why the series is so great. That being the case, it was quite upsetting to some fans when Highschool DxD Season 3 (or High School DxD BorN) of the anime adaptation diverged significantly from the source material. Thankfully, Highschool DxD Season 4, which is known as High School DxD Hero, has fixed that error with Episode 0 of the new anime.
The first two seasons of the Highschool DxD anime covered only four light novel volumes. That’s quite significant since some bad anime adaptations will take an entire novel and somehow squeeze the story down into a single episode. When anime studio TNK created Highschool DxD Season 3, they tried to squeeze three books into a single season, but what really angered the light novel fans was the anime original ending.
The third season, Highschool DxD BorN, greatly rearranged the presentation of the story by pulling elements from later novel volumes. All in all, Highschool DxD Season 3 covered the remaining parts of the Birth of the Breast Dragon Emperor story arc in addition to the opening portions of the Heroic Dragon Oppai arc from Volume 7. Episodes 9 through 12 changed the story completely, resulting in the destruction of the Kuoh Academy school building and had Issei fighting a mind-controlled Rias. The two even confessed their love for each other, which was a massive change for the anime to make. The whole incident with Loki was also not in the light novel.
Highschool DxD Season 4 was produced by anime studio Passione, a relatively new company that’s mostly known for Rokka: Braves Of The Six Flowers and the Citrus anime adaptation. Besides changing the anime’s art style to be like that of light novel illustrator Miyama-Zero, Passione director Yoshifumi Sueda started the fourth season with Episode 0. It’s not merely a recap of the ending of the third season; it literally retconned the last several BorN episodes out of existence.
The activation of Issei’s Juggernaut Drive and the defeat of Shalba Beelzebub is still there, but this new beginning bridges the gap between the third season and Highschool DxD Season 4 by following the story of the light novel. Hopefully, that means the full 13 episodes of the Highschool DxD Hero anime will remain faithful to the light novels.