From MangaHelpers & Kirei Cake:
Vampires now coexist with humans by forming contracts with human beings. Vampires who form these contracts become their pets, serving their every need. They are given pet collars, distinguishing them from wild vampires, which are still very dangerous.
Daichi really wants a pet vampire, but his older sister won’t hear anything of it. One night, a wild vampire princess named Iko breaks into his house! Iko discovered that Daichi’s older sister has the extremely rare blood of Chernobog, which she believes would make her powerful enough to destroy the anti-vampire organization called Abraham once and for all.
However, the blood doesn’t work exactly as Iko expects, and this vampire soon finds herself forced to become the pet of Daichi and his older sister...