Sekai de Ichiban Tadashii Kyuuketsuki no Kaikata

In a world where vampires and humans coexist under the strict terms of a pact forged with "Abraham," a former anti-vampire organization, keeping a vampire as a pet has become a widespread trend. Domesticated vampires are easily identified by their collars, which distinguish them from wild vampires who remain highly dangerous. A teenage boy named Daichi longs to own a pet vampire of his own, but his older sister firmly refuses to allow it. Everything changes one night when a wild vampire named Iko breaks into their home. Iko discovers that Daichi's sister possesses the extraordinarily rare blood of Czernobog, a power she believes will grant her the strength to destroy the humiliating Abraham organization once and for all. However, the ancient blood does not function exactly as Iko anticipated. Instead of achieving ultimate supremacy, the proud vampire finds her plans completely derailed, ultimately forcing her into the very role she despises—becoming the pet of Daichi and his older sister.