Black Cat is a small anthropomorphic black cat that appeared in Kuro's mind. He is a part of Kuro and the one in charge of his power.
The Black Cat takes the appearance of a black cat doll with a long unruly tail, red round eyes, and stitches around his ears and on his mouth. When Mahiru entered Kuro's mind the second time, he is shown to wear a witch hat.
The Black Cat seems to represent Kuro's inner feeling as he always points out what Kuro truly feels whenever the latter tries to hide it.
The Black Cat first appears when Mahiru's consciousness enters Kuro's mind. The Black Cat welcomes Mahiru by congratulating him on his "birthday" and tells the Eve that they are inside Kuro. He motions Mahiru to follow him so he can give the Eve his birthday gift, which Mahiru assumed to be his Lead. He explains to Mahiru that he is a part of Kuro and is in charge of the Servamp's power. He then shows Mahiru a pile of gifts and tells him that he can choose whichever one he like. He warns Mahiru, however, that once he chooses he can't return to this place but he'll become an adult that can protect others. He explains that Mahiru's end will be the same no matter what he choose, though the routes he can take are all different and what's important is that his answer matches who he is.
Battle Against Sakuya, Belkia, and Otogiri
When Mahiru starts to doubt his power and choice, this also affects Kuro, prompting Mahiru's feeling to overwhelm and seep into Kuro. Inside Kuro's mind, the Black Cat is drowning due to Mahiru's overflowing mind, asking Kuro what they should do. He then states that if Mahiru never met Kuro, he wouldn't have become this way and when Kuro does something, the world always falls under misfortune. This causes Kuro to lose control of his power and it manifests into the form of a much bigger version of the Black Cat as it slowly tries to engulf Mahiru. Fortunately, this was stopped by the Servamp of Envy, Jeje, who shoots Kuro several times until he changes back into his cat form.
Captured by C3
When Kuro and Mahiru got captured by C3 and try to find a way out, Kuro reveals to Mahiru that there still many things he hasn't told the Eve but he refuse to tell him because Mahiru will definitely involve himself into vampire business. To Kuro's surprise, Mahiru honors Kuro's wish and decides to trust him, allowing him to keep the story until Kuro himself is ready to tell him. Kuro hides his surprise by feigning disinterest. When separated from Mahiru, the Black Cat appears and points out how pathetic Kuro is for being happy after hearing Mahiru's words and reminds him that C3 makes him remember lots of things. The Black Cat continues to taunt Kuro by telling Kuro that he is nervous of how Mahiru will react if he knew about "that" and how he is getting lost right now. He points out that Kuro is pathetically lost as shown by the fact that his words and actions are all contradictory. The Black Cat then suggests that they break down C3 while they are at the place since they hate C3. All these thoughts clearly disturb Kuro who forcefully pushes it away.
Kuro suddenly becomes a black ball and unable to get out after encounter against Tsubaki's subclass, Higan, so Mahiru takes him to Mikuni's shop where Mikuni's acquaintance, Johann, gives Mahiru a drink that sends him to Kuro's mind once more. The Black Cat tells Mahiru that Kuro doesn't want to see him, but Mahiru doesn't buy it and keeps looking for Kuro until he reaches a door under the desert where the Black Cat is waiting him. The Black Cat changes Mahiru's clothes into a black suit and makes him sit and watch a fragment of Kuro's memories. After witnessing Kuro's memory, the Black Cat muses that the only thing eternal as long as Mahiru is a human is the sin of man. Mahiru then finds himself holding the Black Cat upside down, prompting him to let the cat go. The Black Cat expresses his seeming disappointment that Mahiru came this far, stating that he should just leave Kuro alone. He then suggests that Mahiru pry open one of Kuro's secrets so he may learn many things. He offers to give Mahiru another weapon stronger than broom so he can break the secrets and he should break them all open. However, Mahiru just wants Kuro to let him in so he doesn't need weapon.