Spiral, Vol. 3: The Bonds of Reasoning — Kyo Shirodaira

When one of Ayumu's teachers promises to give him a few clues about the Blade Children, the deceptively cute Rio, one of the Blade Children herself, ruthlessly murders the man in broad daylight. Faced with another brutal murder to solve and one step closer to finding out what the Blade Children really are, Ayumu and Hiyono find themselves the direct targets of Rio's malice. When her murderous plot backfires and Rio finds herself in the hospital, she challenges the visiting Ayumu to a life-and-death gamble, only to find herself face-to-face with Hiyono, who call's Rio's bet and raises her one!