In Blue Exorcist, the human world is called Assiah, and the demon world is called Gehenna ... Illuminati with the Foundation ...

Although the founders of the True Cross Order came from different backgrounds, they played a vital role in its foundation.

Rin is definitely more about charging in and throwing his blue fire around like crazy, but Yukio is good at thinking at least a few steps ahead, as seen here when he tries making a deal with Hachitaro ...

Blue Exorcist season 4 episode ... the realm that birthed the Demons. In other words, it was Satan himself, the father of Rin and Yukio. Furthermore, the Blue Night happened on the day Satan ...