Iori Mutou is a "newly awakened Zerozaki" whose peaceful life is forever changed when she is confronted by mysterious individuals out for the Zerozakis and mistake her for one of them. Up until that point she lived with her older brother, sister, mother and father until her family was hypnotized and attacked her.
She is a member of the Zerozaki Family, not really a family but a collection of humans who have an affinity and quality for murder, and loosely joined together out of loneliness. Though usually a Zerozaki is irresistibly drawn to acts of violence and evil, Iori had no such instincts which has peaked the interest of Soushiki Zerozaki who wants to see if she can continue her life or succumb to the ways of the Zerozaki.
For some reason Iori has always felt the need to run away from her problems instead of facing them. She admits to herself that she feels the need to run away from a non-existent inner demon, which leads her to sometimes neglect important things such as homework. She appears to be modest as she wears shorts underneath her skirt.
Being a Zerozaki means that she has an innate affinity towards murder, especially a Jack-the-Ripper like Cutthroat style of murder and yet this nature was completely dormant within her most of her life. It was not until her life was in danger that this nature awakened and she killed somebody for the first time - her brainwashed friend - it was than that her innate nature started to surface. Even after only just meeting somebody she got the immediate urge to kill him, despite not knowing why.
Even when seeing the life-or-death match between Soushiki Zerozaki and his opponent, watching the decapitated heads fall from the sky and see blood, she was not scared, but rather excited. It appears that her inner Zerozaki has not yet fully awakened which is the reason why she has not killed anybody outside of self-defense.