Keys play an important role in the world of Blue Exorcist. Magic keys allowing for instantaneous transport between two points are a staple tool of the True Cross Order and very valuable, requiring Exorcists to take good care of them. Despite their versatile ability, there is a limit to the use of the keys, being unable to transport even as few as six people over a significant distance. Other types of magic key exist, but they do not appear to be as prevalent.
A key that allows access to the Opera Court House via the Door of the Accused. Conditions for earning this key are unclear, but it seems that the Paladin at least holds one.
A key that allows access to the Exorcist Supply Store, Futsumaya, located in True Cross Academy. It is only given to fully qualified exorcists and access to the store is also restricted to them.
A Kamikakushi Key (神隠しの鍵Kamikakushi no Kagi) allows something to be hidden in any given location. It was used by Shiro Fujimoto to hide the Kouma Sword for many years. It can also be used to travel to the past.
Key of Eternity
The Key of Eternity is a key that allows limitless travel. Mephisto Pheles gives the key to Amaimon to allow him to travel freely in and out of the school.
A key granted to Middle Second Class Exorcists. It allows the user to pass between doors of the same type, meaning one could enter one train door and come out another of the same design.
Tsukumo Kamiki's Key
This key is linked to Tsukumo Kamiki. A get-well present for Izumo Kamiki from Nemu Takara's employer, it opened a door to the adoptive home of her younger sister. It is unclear if the key only links to this location, or it specifically follows where Tsukumo is at the time.
A key that only appears in the anime. It was kept by Mephisto Pheles, linking to the mountain cottage owned by Yuri Egin. It is eventually passed on to Rin and Yukio so that they can visit the place where they were born.
Tsukumo Kamiki's key seems to be designed aesthetically for her, having a crescent moon for the handle and her name containing the kanji for the same.