Ryohei's Killer Points
Killer Points are loopholes in a person's body created when blood circulates. Muscles in the body retract and cells alter, resulting in the creation of these points. These are spots which move along with the body's movements. When a Killer Point is hit (even with the lightest attack), the resulting pain can prove fatal to the targeted individual. With each successful power blow to a Killer Point, the affected spot will grow larger (possibly a side-effect, such as blood clotting), eventually turning the entire body into a single Killer Point.
Kouyou Aoba is the only known individual who can see Killer Points. In fact, he possesses extraordinary vision that allows him to see these "weaknesses" in his enemy's body. This ability was first hidden by his glasses, which initially seemed like normal glasses, but were revealed to be glasses that obstruct the wearer's view; Kouyou revealed that this was to seal off his advanced vision, which also consumes more energy the more it is used.