Servamp

Mahiru Shirota is an ordinary high school student who enjoys the straightforward. On his way home one day, he comes upon a black cat and chooses to adopt it, naming it Kuro. He is shocked to learn the following day that the black cat is actually one of the seven vampires that stand in for the Seven Deadly Sins. He goes by the name Sleepy Ash, stands in for Sloth, and when exposed to sunlight, transforms into a cat. Mahiru unintentionally creates a temporary contract with Sleepy Ash by giving him a name, a cat bell, and using that name when addressing him. Once Sleepy Ash consumes his blood, he becomes Mahiru's vampire servant known as "Servamp," and Mahiru becomes his master known as "Eve." This completes the pact. After that, Tsubaki, a vampire who claims to be the Servamp of Melancholy and the unidentified eighth brother of the other Servamps, meets Mahiru and Sleepy Ash, now Kuro. While Mahiru chooses to unite them to battle Tsubaki and his group as a whole, Tsubaki aims to wage an all-out war against his six brothers and sister.