Yoru

A girl cloaked in nocturnal darkness, whose existence is revealed for the first time at the story’s beginning.
Unlike other shadow monstrosities, she communicates through speech.
Her hatred toward the protagonist group is a bottomless abyss. Declaring war with her creed—"Plunge the world into lightless darkness"—she is devoid of all whimsy.
Her true nature, motives—all remain shrouded in mystery.

Translated from official website