Yui Kurata

Yui is a special mage, so she can only communicate with others through dreams. And also, lives in the dungeon underneath the academy.

History
Yui is a mage who lives in the other side of the world, the dream world. And it wouldn't be an exaggeration to call that her thema. So staying asleep is apart of her thema's research. Which is also exactly why she's been sealed. Her power is in the cardinals class, which means within the school, she's the next one below the headmaster. Without her restraints in effect, her magic power causes danger, i.e. the entire world falling asleep. And with the entire world asleep that will cause its destruction.

Story
Arata met her when she saved him from being killed by Akio to stop the breakdown phenomenon. She pulled him into the dream world at the last moment. There she introduced herself, and explained to him what archives and themas were.

Because Arata met with her, the power of the magic king affected her. Causing the entire school to go to sleep.

Trivia

 * Yui rarely ever shows up at school.