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Yamane Byouinzaka (病院坂 病子)She is Houko's younger sister and author of the killing good called "The Injection of Eternal Sleep" that can kill people either by being injected or being knicked. She's often very unpredictable and constantly on the edge when her lust for blood is not satisfied which is why she is very much depended on Houko as her "Instead." Her first victims are her parents, in which Houko took the blame for it so as to keep her from being incriminated.
It wasn't clear why she and her sister, Houko refer to themselves as "twins" when they're neither identical nor fraternal, however it is hinted that it was the bond that they have for each other that they refer to themselves as "twins."