Sekai de Ichiban Tadashii Kyuuketsuki no Kaikata

Vampires now coexist with humans, by the terms of their contract with the former anti-vampire organization "Abraham." This contract determines that vampires are to be human's pets, and keeping a pet vampire has become very popular. Their pet collar distinguishes pet vampires from wild vampires, and the wild ones are still quite dangerous.

A teenage boy named Daichi really wants a pet vampire, but his older sister won't hear of it. One night, a wild vampire named Iko breaks into his house! The vampires found that Daichi's sister has the extremely rare blood of Czernobog, which she believes will make her powerful enough to destroy the demeaning "Abraham" organization. However, the blood doesn't work exactly as Iko expected, and the proud vampire soon finds herself forced to become the pet of Daichi and his older sister...