Sekai de Ichiban Tadashii Kyuuketsuki no Kaikata

Vampires live alongside humans under an agreement with the former anti-vampire group "Abraham," which requires them to serve as human pets. Pet vampires wear collars to distinguish them from dangerous wild vampires. Daichi, a teenager, wants a pet vampire, but his sister refuses. A wild vampire named Iko invades their home, drawn by the sister’s rare Czernobog blood, which Iko believes will empower her to dismantle the Abraham organization. The blood does not function as anticipated, leading Iko to reluctantly become a pet under Daichi and his sister.