In his human form, Kuro has shoulder-length blue hair with bangs that fall over his eyes, which are red and perpetually half-lidded, giving him an expression of boredom. Prominent dark bags rest beneath his eyes, further emphasizing his lethargic demeanor. Like other vampires, he possesses pointed teeth.

Kuro’s signature outfit consists of a blue hooded jacket, slightly darker than his hair, trimmed with black fur that resembles the fur of his cat form. A patch marked with the Roman numeral I decorates his left shoulder, signifying his status as the eldest of the Servamps. He wears the bell given to him by Mahiru Shirota the symbol of their contract around his neck, though it is usually hidden beneath his clothes. His jacket is often worn open, revealing either a plain black or white shirt underneath, and it features chest pockets on both sides. The hem of the jacket splits into multiple thread-like strips, shorter in the front and extending longer toward the back. His attire is completed with skin-tight white pants secured by a red belt, usually concealed, and long black boots that nearly reach his knees.

Centuries earlier, Kuro wore a long black hooded jacket paired with white trousers, a dark gray suit underneath, and a loosely worn scarf.

In his cat form, Kuro has sleek black fur, red eyes, and Mahiru’s bell fastened around his neck with a red leather collar. His tail splits into four distinct ends, echoing the thread-like strips that hang from the back of his jacket in his human form.

Before becoming a vampire, Kuro appeared with light blond hair and blue eyes, without the heavy under-eye bags that later came to characterize his appearance.

As the Servamp of Sloth, Kuro embodies lethargy and avoidance. He spends much of his time sleeping, idling, or playing video games, and openly avoids responsibility by refusing to make decisions, claiming he does not want to be blamed for failures or disappoint anyone. He is a pacifist at heart and a self-described shut-in, preferring to stay indoors rather than engage with the outside world. Sensitive to changes in temperature, he often remarks on the delicate nature of vampires. Kuro rarely feeds on Mahiru Shirota’s blood, only doing so when preparing for combat.

Despite his comical laziness and frequent arguments with Mahiru, Kuro possesses a darker, more volatile side. Mentions of his past particularly his role in killing the Servamps’ creator—shift his behavior from detached to aggressive. Burdened by guilt, he views himself as a monster and rejects further knowledge about his kind, believing it would only confirm his self-loathing. He refuses to create subclasses by turning humans into vampires, reasoning that no one would willingly choose such a fate and that vampires should not exist at all.

Mahiru’s sincerity gradually influences Kuro, challenging his desire to run from responsibility. After Mahiru confronts him directly about his past and reassures him that he is not alone, Kuro resolves to face his history and take a more active role in opposing Tsubaki. Since then, he has shown greater determination and willingness to fight.

In contrast to his heavy guilt, Kuro often takes pride in his cat form, frequently describing himself as “cute” or “super cute.” He exploits this appearance to charm Mahiru’s classmates into giving him food or to defuse tense situations by playfully asking others to relax in his presence. However, these attempts rarely succeed, as those closest to him are well aware of his true nature, with the exception of Licht, who is notably fond of small animals.

Sleepy Ash is the first and oldest of the eight Servamps. As a human, he died of unknown causes and was resurrected as a vampire by their creator, a figure known as “The Count.” His transformation horrified him; during the day, he could maintain his human form only by remaining in the shadows, while at night he transformed into a lion driven by an insatiable thirst for human blood. His fear and self-loathing deepened after he lost control and destroyed the town where he once lived.

Following the destruction of his hometown, Sleepy Ash wandered aimlessly until he heard rumors of a werewolf residing in a distant village. Seeking help, he traveled there and encountered Neugear Hartevelt, known as Gear. Sleepy Ash pleaded with Gear to free him from his uncontrollable beast form and bloodlust. Gear performed an exorcism ritual that separated Sleepy Ash’s destructive power, transferring it into a rose. With this power removed, Sleepy Ash no longer transformed at night. At his request, Gear disposed of the rose, severing Sleepy Ash’s connection to the beast.

Centuries later, the organization C3 approached Sleepy Ash with a request to kill the Servamps’ creator. He convened his siblings to debate the decision, which ultimately became contentious. They agreed to resolve the matter by majority vote, with Sleepy Ash casting the deciding vote. Convinced that their creator’s experiments posed a grave threat and would result in more beings like themselves, he chose to accept the mission and volunteered to carry it out personally. This choice caused a severe rift with his younger brother, Lawless, and strained their relationship for many years.

Before departing for Japan, Sleepy Ash met with Gear one last time to explain his decision. Upon finally confronting the creator after centuries, he was unsettled to find him still alive. Though shaken, he reaffirmed his belief that the experiments were too dangerous and killed his creator. However, the act left him plagued with doubt. Sleepy Ash began questioning whether his choice had been right, and his uncertainty and guilt over this decision continued to haunt him.

Transformation
Following his resurrection as a vampire, Kuro would involuntarily transform into a giant black lion during the night, while remaining in human form during the day only if sheltered in shadows. After Gear separated the core of his destructive power, Kuro gained the ability to freely shift between his cat form and human form. In his cat form, he retains the ability to speak. When exposed to direct sunlight, he automatically reverts to a cat. Kuro can still assume his lion form, but doing so requires an enormous expenditure of energy.

Servamp Bond
As the Servamp of Sloth, Kuro forms a contract with his Eve, Mahiru Shirota. When drinking Mahiru’s blood, the chain of their bond becomes visible, and Kuro’s strength increases. The magnitude of this enhancement depends directly on Mahiru’s resolve: strong determination yields significant power boosts, while hesitation provides little to none.

Claws
Kuro can extend his nails into elongated, razor-sharp claws capable of cutting through both humans and other vampires with ease.

Speed and Flight
He exhibits exceptional speed and agility, able to cover distances in an instant and leap great heights. Kuro has also demonstrated limited flight capabilities.

Endurance
As an immortal vampire, Kuro cannot die through conventional means and recovers rapidly from injuries. He has continued fighting even after being stabbed through with a sword or attacked repeatedly with knives. However, he can still be incapacitated temporarily, as seen when he was rendered unconscious after being shot in the back by Envy’s guns.

Inner Power
When losing control of his suppressed power, Kuro becomes significantly stronger, faster, and more violent, attacking without restraint. In this state, he has been shown to defeat opponents brutally, such as when he nearly killed Berkia, one of Tsubaki’s subclasses. His loss of control can also endanger allies, as seen when he unintentionally injured Mahiru by slashing his face and biting his neck during a frenzy, before being stopped by Mahiru and Envy.

Jacket Tails
Kuro can manipulate the threadlike tails of his jacket as hardened appendages, using them offensively in combat. He is also able to generate black spikes from the garment, which can impale and restrain his enemies.

Mahiru Shirota (Eve)
For centuries, Kuro lived without an Eve until he was picked up by Mahiru Shirota, who named him and became his partner. At first, Kuro remained with Mahiru only because of the contract, often acting reluctantly and claiming that most of his actions were the result of Mahiru’s insistence. Their opposing personalities frequently led to arguments, with Mahiru’s determination clashing against Kuro’s apathy. Over time, however, Mahiru’s sincerity and resolve began to influence Kuro. Though he continued to hide behind his laziness, Kuro grew genuinely attached to Mahiru and valued his trust. Their bond deepened after Mahiru helped Kuro confront his past, at which point Kuro openly acknowledged Mahiru as his partner.

Lawless
Kuro’s relationship with Lawless was marked by long-standing hostility. Their rift began when Kuro chose to kill the Servamps’ creator, a decision Lawless strongly opposed. Lawless declared he would never forgive Kuro, and their meetings were often marked by mocking and open animosity, sometimes provoking Kuro into violent outbursts despite his usual reserved nature. Although Kuro understood Lawless’s hatred, he did not attempt to excuse his actions. After deciding to face his past, Kuro apologized, thanking Lawless for not forgiving him immediately, as it forced him to confront his guilt. Their relationship improved afterward, and the two began spending more time together, even teaming up to irritate Mahiru with playful antics.

Tsubaki
Tsubaki harbors deep resentment toward Kuro for killing their creator. Unlike the other Servamps, Kuro was unaware of Tsubaki’s existence and initially did not acknowledge him as a sibling. Tsubaki, however, had been searching for Kuro since witnessing his lion form during the death of their creator. Their first encounter was violent, with Tsubaki attacking Kuro mercilessly. The truth of their relationship became undeniable after Tsubaki destroyed Lawless’s contract item and revealed his knowledge of Kuro’s role in their creator’s death. Concluding that Tsubaki’s hatred was fueled by this event, Kuro accepted responsibility and resolved to stop him while also seeking to understand him, determined not to repeat the mistakes of the past.