Kokushibo

Kokushibo is a member of the Twelve Demon Moons, holding the position of Upper Moon One.

Before becoming a demon, he was a Demon Slayer named Michikatsu Tsugikuni.

Appearance
Kokushibo is a tall man with long black hair that he keeps in a ponytail. He has three sets of eyes on his face, with red eye whites and yellow irises. His middle set of eyes features the mark of Upper Moon One, and red markings resembling flames can be seen on his face and neck. He wears a patterned kimono with a black hakama on his waist. On his waist, he carries a sword that has eyes in the space between the tsuka wrapping of the handle of his sword. The guard and blade were also later revealed to have eyes and veins.

Personality
Kokushibo is reserved yet shrouded in mystery. At the meeting of the Upper Moons, he is punctual, rule-abiding, and places high value in the hierarchy of the Twelve Demon Moons. He is also humble, as he did not hesitate to admit his failure or complain of any difficulties, instead acknowledging his opponent's strength. He has also displayed a strong sense of loyalty towards Muzan Kibutsuji, with most of his actions being done with the sole purpose of fulfilling Muzan's goals.

Despite his outward reservation and humility, he is also rather cold; his words can be harsh, especially when reprimanding individuals. His threats come off as nothing short of cruel and overwhelmingly serious, and he demands absolute obedience.

Kokushibo has a complex relationship with his human past. He shows something of warmth towards his descendant Muichiro Tokito, commending his skills and resolve, and moves to convert Muichiro into a demon to spare his life. At the same time, he refers to Muichiro and his abilities as the natural result of his "cells" being passed down. As a result, it is unclear whether he has any true attachment, or if it is the result of narcissistic pride. Seeing Sanemi Shinazugawa and Genya Shinazugawa together&mdash;two Demon Slayer brothers&mdash;makes him nostalgic, and dueling Sanemi reminds him of his days dueling the first Wind Pillar to improve his skills.

In his duel against Gyomei Himejima, his banter suggests a fear of death. When Gyomei's marks appear, Kokushibo preemptively bemoans the loss of a talented fighter and urges Gyomei to become a demon to continue honing his skills. He seems genuinely surprised when Gyomei vehemently rejects his offer and calls his mentality ridiculous.

Human Life
Born in the Sengoku Period, Kokushibo's birth name was Tsugikuni Michikatsu. At some point in time, he learned and mastered the Breath of the Moon Breath Style and joined the Demon Slaying Corps. During that time, he trained alongside a Breath of Wind user and polished his sword skills together with them. Later in his human life, he had a child who would go on to continue his bloodline centuries later.

Demon Life
At some point, for reasons unknown, he became a demon, took on the name Kokushibo, and joined the Twelve Demon Moons, eventually obtaining the rank of Upper Moon One and remaining at that rank for hundreds of years. Having fought numerous Demon Slayers across the centuries, he encountered one Demon Slayer about 300 years prior to the present with an ability similar to Genya's&mdash;devouring demons to enhance his own strength. That time, Kokushibo slew him by cutting his torso into two.

Red Light District Arc
Kokushibo was summoned to a meeting of the Upper Moons, arriving before all of his colleagues. He silently sat facing a wall, only speaking when his presence was acknowledged by the Biwa Demon, and revealing that Kibutsuji Muzan had arrived. Muzan revealed the death of Gyutaro, berating Kokushibo and his fellow Upper Moons for being unable to destroy the Ubuyashiki family and failing to find the Blue Spider Lily. Kokushibo admitted that he had nothing to say in his defense, as Kagaya Ubuyashiki had cleverly hidden himself.

After Akaza brutally destroyed Doma's head, Kokushibo covertly sliced off the former's hand, stating he had gone too far. He explained to Doma that he had not intervened for his benefit, but for the preservation of the hierarchy of the Upper Moon system. He then told Akaza to arrange for a blood battle if he wanted to take Doma's position, asking him if he understood what he was being told. Akaza acknowledged this fact, promising to kill the high-ranking Upper Moon, which Kokushibo wished him luck in before departing.

Dimensional Infinity Fortress Arc
During a meeting of the Pillars, Amane Ubuyashiki speculates Kokushibo may have been one of the original Breath users during the Sengoku Era, the group that was one step away from ending Muzan.

Kokushibo is present in a flashback of Kaigaku, depicting the time just before Kaigaku following his defeat at the hands of Kokushibo. Kokushibo then offers his own blood and the power that Kaigaku craves if "that man" is willing to accept him as an ally. And as he does, he threatens that if a drop of blood is spilled, the human will be beheaded.

Following Akaza's death, Kokushibo is seen alone. Murmuring how Akaza had abandoned his path to reach further heights and how he had failed to defeat Kokushibo, he reprimands how weak Akaza is. Simultaneously, he reaches for his katana, and instantly slices the surrounding stone pillars to pieces in a single stroke.

Abilities and Powers
Kokushibo is an exceptionally powerful swordsman, considered the strongest of all Twelve Demon Moons and second strongest demon in the series, just behind Muzan Kibitsuji. As one of the oldest demons in the series, he has battled numerous Demon Slayers and has obtained a large pool of experience and knowledge of combat, easily deciphering the techniques and skills of at least Two Pillars and overwhelming them in battle. As a breath user, he possess superhuman levels of strength and reflexes that are further enhanced by his demon physiology.

His six pairs of eyes grant him superhuman kinetic vision similar to Kanao Tsuyuri's, allowing him to peer into his opponents' insides, such as their muscles, organs and even their cells. Kokushibo was able to immediately discern the skill level of Muichiro, Sanemi, and Gyoumei with one look, identify that Muichiro was his descendant, and he was able to see that Genya consumed demons to gain strength by looking into his cellular structures.

These overall abilities and skills allowed him to defeat two Pillars with minimal effort and then fight on par against Gyomei, who is currently the most powerful Demon Slayer in the Demon Slaying Corps. Even when Himejima and Sanemi awaken their demon slayer marks which are supposed to increase one's physical abilities further, he still capable of holding his own against the both of them simultaneously.

Breath Styles
Breath of The Moon (の) - Like Kaigaku, Kokushibo is a Demon with an adept Demon Slayer Breath style. His Breath of the Moon is one of the most dangerous sword styles displayed thus far. Sanemi noted that each and every slash and cut he makes using this breath style is surrounded by Chaotic crescent moon like blades that constantly change size and length, increasing the range of his already powerful attacks.
 * 1) First Form: Dark Moon - Evening Palace (ノ ・の): Kokushibo draws his sword and slashes swiftly in a single motion, like with all Breath of the Moon techniques, numerous chaotic crescent shaped blades originate from the slash. This technique resembles Iaido.
 * 2) Second Form: Pearl Flowers Moon-Gazing: (の ノ)
 * 3) Third Form: Loathsome Moon: Chains:  (ノ ・り): Kokushibo swings his sword rapidly in two crescent slashes, from which a storm of smaller crescents spread. This technique causes huge destruction in a small area.
 * 4) Fourth Form
 * 5) Fifth Form: Moon Spirit Calamitous Eddy (ノ): Kokushibo makes multiple curved slashes layered over one another resembling a rising vortex. Numerous chaotic blades originate from these slashes. Kokushibo performed this attack without swinging his blade.
 * 6) Sixth Form: Perpetual Night, Lonely Moon - Incessant (ノ ・): Kokushibo releases a wild storm of slashes in multiple directions. This technique was powerful enough to not only slice up multiple pillars around him but also overwhelm the Wind Pillar Shinazugawa Sanemi.

Flesh Sword - Kokushibo reveals that he had detached some of his own flesh and shaped it into the Katana that he uses in battle. The blade possesses numerous eyes in the blade, handle, and guard, allowing him to have increased range of vision while using it in battle. Also, since the blade is created from his flesh, the blade can easily regenerate if destroyed or chipped. This ability is shown when the blade returns to its original shape in his battle with Gyomei Himejima. Later, Kokushibo also displayed the ability to freely manipulate the form of the blade, growing three additional blades from the original blade to increase its reach.

Trivia

 * Kokushibo bears a striking resemblance to the first Breath of the Sun user. A panel in chapter 128 even raises the possibility that the two are siblings(assuming they are Kokushibo and the first Breath of the Sun user) given the striking resemblance of the two demon slayers.
 * Kokushibo is chronologically the first Demon Slayer to become a Demon.

Quotes

 * (In response to Muzan's reprimand) "I... have nothing... to say... Ubuyashiki... cleverly... hid... himself..." Chapter 98, page 15.
 * (To Akaza) "You... went too far..." Chapter 99, page 3.
 * (To Kaigaku) "Be thankful for the blood... You are not allowed to spill even a single drop on the ground... for if you do... your torso and head shall have a sorrowful parting." Chapter 145, page 5.
 * (In response to Akaza's death) "You opened a path... to reach further heights... and you abandoned it... Very weak." Chapter 157, page 5.

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