Is Blue exorcist really based on a slush puppy?
In an interview with Animenewsnetwork, he said that he was inspired by a Grimm fairy tale:

What were your chief inspirations when creating Blue Exorcist?
Kato: It originally was inspired by a Brothers Grimm fairy tale, where brothers combat monsters.  Then, I tried to make the story flip several times, and settled down into a simple structure of demons versus an exorcist.

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