Blue Exorcist is an action-packed supernatural series with an anime adaptation that is typically regarded as wasted potential due to being handled poorly. The anime's first season aired in spring 2011 with 25 episodes and got a similar treatment to Fullmetal Alchemist (2003) with an ending that did not follow the manga at all -- becoming anime-only to tie up the main story. Due to these circumstances, it was done in a rushed and lackluster manner, which only succeeded in disappointing fans.
Since then, the manga retained a cult following while the anime eventually received a second season titled Blue Exorcist: Kyoto Saga and aired nearly six years later in winter 2017. This new season consisted of only 12 episodes and wholly ignored the last several episodes of the first season to faithfully continue the manga.
Blue Exorcist, written and illustrated by Kazue Kato, takes place in a world where there are two dimensions; Assiah, the world of humans, and Gehenna, the world of demons ruled by Satan. Rin Okumura is a troublesome teenager whose life gets turned on its head when he discovers that he and his younger twin brother, Yukio, are the sons of Satan. While Satan wants Rin to give in to his demonic side to conquer Assiah together, he is against such plans. Instead, he begins his training to become an exorcist to one day defeat Satan and defend Assiah alongside his brother.
The manga began serialization in April 2009, published in Shueisha's Jump Square with an English translation available through Viz Media. The manga is still ongoing, with 26 volumes being released thus far. Both seasons of the anime were produced by A-1 Pictures Studio and are still available to stream in both subbed and dubbed on Crunchyroll.
Blue Exorcist is filled with fascinating lore, unique magic and an exciting premise, and the series had the makings and the fanbase to ensure the anime was successful. Unfortunately, 10 out of the first season's 25 episodes were purely filler, with another five episodes containing additional filler content. The original story was muddied with the season finale throwing a lot of the story and character development out the window. Though despite its messy conclusion, A-1 Pictures released a film in 2012 titled Blue Exorcist: The Movie featuring the same beloved characters in an original story.
Luckily, due to the manga's popularity, the series eventually received another season, but damage control had to be done to fix the disastrous finale of the first season. The second season followed the manga's canon storyline, ignoring all filler content from the previous season for a more faithful adaptation. However, this became jarring to fans who haven't read the manga, trying to piece together the main story by ignoring a good portion of the first season.
While there is plenty of manga to make a third season, the inconsistency in the anime adaptation of Blue Exorcist has likely ruined all chances for the anime to continue despite its massive cult fanbase. If this weren't the case, the series probably could've enjoyed the same success as other shonen series like Black Clover.
Unfortunately, at the time of its airing, anime-only endings were not uncommon. If an anime was ending before its corresponding manga and it was unknown whether it would be renewed for another season, the production team would often come up with an anime-only ending to offer some conclusion. Though this concept has mostly died off, such fillers usually take on a different voice from the original creator and vary in quality and are rarely favored by fans.
Overall, Blue Exorcist was cut shockingly short before it had a chance and was done an injustice with its poor treatment. Due to an inconsistent narrative, neither the story nor the characters were allowed to be fleshed out, leaving the Blue Exorcist anime to fade into the background behind other, more polished, shonen series. At the very least, with the manga still ongoing, fans are best sticking with that to see how the remainder of the story plays out.