Koku is a young man distinguished by his heterochromia; his right eye is grayish-black, while his left eye is dark teal. He has long black hair and typically dresses in casual clothing, often wearing blue shirts paired with white capri pants. His quiet and observant nature is reflected in his reserved expression.

In his black-winged king form, Koku undergoes a striking transformation. His left eye becomes more intricate, displaying three red pupils, and his skin darkens with small blue, scale-like flecks. Two black wings emerge from his back, and he can transform his left arm into a blade of blue steel at will, using it as a weapon. His left leg, transplanted from Izanami, shares similar metallic properties.

Later in the story, after reaching the Moby-Dick, both Koku’s and Yuna’s hair turns white. Following the events at the Jet Black Epitaph, their hair color reverts to normal. Aside from a brief comment by Laica describing Yuna’s change as a “silver nuptial coloration,” the phenomenon remains unexplained.

Koku is generally quiet, polite, and reserved, often showing small acts of kindness, such as bringing red bean buns to his employer’s family. Though he is highly skilled in repairing violins, he does not play them himself and tends to keep a low profile in his daily life.

In contrast, his identity as Killer B reveals a much more aggressive and ruthless side. Focused entirely on achieving his goals, Koku shows little hesitation or mercy when dealing with his enemies, including those perceived as common criminals. This side of him operates with cold precision, driven by a strong sense of purpose.

Koku was one of several genetically engineered children created at the Jaula Blanca Institute as part of an experimental project to develop beings with extraordinary powers. At birth, he possessed two eyes capable of memory manipulation, although his right eye was later removed and implanted into Minatsuki for experimental purposes.

Koku formed strong bonds with several other children at the institute, including Yuna, his closest companion and love interest. Their lives were upended when Gilbert Ross orchestrated a brutal attack on Jaula Blanca, resulting in the deaths of many children and researchers, including Dr. Heath Flick, one of the few adults who cared for them. Following the massacre, Koku and the surviving children were scattered, with Koku dedicating the next seven years to searching for Yuna, believing her to still be alive.

While living under a new identity, Koku worked as a violin craftsman in Cremona City, employed by Lily Hoshina’s father. However, he continued operating in secret as “Killer B,” targeting those connected to the events of the past, all while trying to reunite with Yuna and uncover the truth behind the conspiracies surrounding Jaula Blanca.

Koku possesses a wide range of superhuman abilities that make him one of the most formidable beings in B: The Beginning.

Memory Manipulation: Through his left eye, Koku can alter and manipulate the memories of others, effectively allowing him to brainwash individuals. Although powerful, he rarely uses this ability, honoring a promise made to Dr. Heath Flick not to use it against the children at the Jaula Blanca Institute. It is implied that Koku originally possessed this ability in both eyes, but his right eye was removed and implanted into Minatsuki during an experiment.

Expert Combatant: Koku is an exceptionally skilled fighter, capable of moving at speeds faster than the human eye can perceive. In battle, he favors his limb blades, particularly the one formed from his left arm.

Regeneration: Koku demonstrates an accelerated healing factor, allowing him to recover from severe injuries, including surviving the explosion that destroyed the Moby Dick airship at the edge of space. However, his regeneration has its limits; for example, the damage to his wings sustained in the explosion does not heal immediately.

Immortality: Koku exhibits a degree of immortality, showing no signs of aging and immunity to diseases.

Superhuman Speed: Koku’s speed allows him to move faster than human perception, easily bypassing security systems and dodging bullets during combat, provided he can see them coming.

Body Part Absorption: Koku can absorb body parts from other enhanced beings, inheriting their unique powers and traits.

Blue Steel Limb Blades (Lohengrins): After absorbing the left arm of Kirisame and the left leg of Izanami, Koku gains the ability to transform these limbs into blades made of blue steel. These weapons are highly durable, capable of breaking conventional materials like gold and steel, and are lethal to Reggies upon contact.

Flight: True to his title as the “Black-Winged King,” Koku can fly using the black wings that emerge from his back. His flight speed allows him to ascend as high as the upper atmosphere.

Disguises: Koku demonstrates considerable skill in crafting disguises, using them effectively to infiltrate locations. Notable examples include posing as a waiter at a party and impersonating RIS medical staff.