Rei Kuroki

A classmate of Akane and her friends. She is mysterious and is never shown to interact with other people.
She also seems to have a connection to the Alone and could be called an enemy.

Rei comes from an another world where she lost everything dear to her. There, a crow made contract with her that in order to get her world back to its normal state, she has to destroy the Manifestation Engine using Alones. Rei has a mark of feathers which indicate the amount of arrows she can still fire, and when the last arrow is fired, she disappears completely if the Manifestation Engine isn't destroyed.