Ketsuki (血飢, lit. "Blood Thirst"), also known as Bloodthirsting, is the vampiric condition which affects most Vespers and Vhouls. Bloodthirsting is the greatest complication of the subjects of Project ESPer.
Vespers, Vhouls, and Ochigami thrive off the vitality of others. Vitality is often spoken of in two terms; blood and spirit. Blood is a major factor in the creation of a Vesper, which takes one part Angel and binds it to a human soul, which is then sealed into a human body. Depriving a Bloodthirster of their chosen form of vitality does not kill the Bloodthirster. Instead, it drives them to a frenzy where they kill without impunity.