Tekki Kaname

She's the hitman parter of the demon Riderk. They team up with Shouya and Armelina early in the series. At an all girls highschool she's a model student everyone looks up to. But she's actually a ruthless killer and currently it's unknown why she became a hitman. In a flashback it's revealed she was abused as a child by her mother which could explain her ruthlessness.