Kuro is a Servamp, a vampire bound as a servant to a human through a blood contract. He initially appears as a stray black cat, but his true vampire nature emerges after forming a contract with Mahiru Shirota, who names him. This contract immediately plunges Kuro into conflicts involving other Servamps and supernatural battles.

Known by the alias "Sleepy Ash," Kuro is one of the seven Servamps representing a deadly sin, though the specific sin is not detailed. His existence and actions are directly tied to Mahiru, their bond dictating their shared involvement in ongoing conflicts among non-humans.

Following a confrontation with the eighth Servamp, Tsubaki, Kuro and Mahiru experience a brief period of normalcy. This peace shatters with an unnatural summer snowfall, caused by the breaking of Servamp Lawless' contract item. This event forces Kuro and Mahiru to investigate the Alicein Mansion alongside allies, where they encounter resistance and the household's denial of Mikuni Alicein's existence.

Throughout these events, Kuro's role remains intrinsically linked to Mahiru's actions and decisions, reflecting the dependency of their contract. Investigating the Alicein Mansion's mysteries and the weather anomaly becomes a central part of their ongoing challenges.