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 pathetic.

Human Life
Born in the Sengoku Period, Kokushibo's birth name was Tsugikuni Michikatsu. At some point in time, he learned and mastered a particular Breath Style (potentially the Breath of the Moon style he uses even as a demon) 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.

Some time before becoming a demon, he had a child who would go on to continue his bloodline centuries later. It is implied that before the age of 25, he developed a particular birthmark that significantly increased his fighting ability, but also doomed him to die when he reached 25 years of age.

Demon Life
At some point, perhaps out of fear of death, he became a demon and took on the name Kokushibo. Because he was originally a Breath user, it took him three days to suppress his Breath techniques and fully transform into a demon. Eventually, Kokushibo became Upper Moon One of the Twelve Demon Moons, remaining at that rank for hundreds of years while his original name was lost to history.

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. It is implied that this Demon Slayer's ability wasn't as adapt as Genya's give that Kokushibo slew him just 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.

Some time later, Muichiro Tokito is transported to the same room as Kokushibo. Kokushibo then comments: "You came Demon Slayer. For some reason, hm? You, you seem familiar". He then deduces that Muichiro is a descendant of a child he left behind after becoming a demon. He also deduces that his human surname of Tsugikuni has died out over the generations. After Muichiro calms himself down and performs various Styles from the Breath the Mist, his Demon Slaying Mark appears. Kokushibo then complements how accomplished Muichiro is as a swordsman at such a young age. Kokushibo states that it must be due to the powerful blood he passed down, although Muichiro dismisses such claims. After some more battling, Kokushibo manages to cut off Muichiro's arm. Muichiro quickly tightens his arm to stop the bleeding, greatly impressing Kokushibo. However, he is effortlessly pinned against a pillar. Kokushibo then offers Muichiro the opportunity to become a demon who is faithful to Muzan. Stating that this would not only heal his arm, but also offer him great power.

This discussion is however interrupted by Genya shooting his Nichirin Gun. Kokushibo easily avoids the gunshot, appearing behind Genya and then proceeding to cut off both his arms and his torso in half, commenting that: "Hmm, I see. You're the one who's been eating demon's, huh?" However, he realizes that Genya is unlike the other imitation demon from 300 years ago and that the way to kill him is the same way regular demons are killed, by chopping off their head. Before he has the opportunity to do this, Genya is saved by his brother, the Wind Hashira Sanemi. After Genya apologizes for everything he's done to Sanemi, Kokushibo states that: "Hmm, brothers and demon slayers, that brings back memories...". Given this and his appearance, it is heavily implied that he might be the brother of the man who passed the Dance of the Fire God down to the Kamado family.

Sanemi and Kokushibo then begin their intense duel, with Kokushibo being slightly taken aback at the skill of Sanemi. However, he retains his composure and states that: "I couldn't survive if my senses hadn't been honed from long experience." This goes on for sometime, with Kokushibo even stating that long ago when Japan was at war, he gained a lot of experience fighting against a similarly strong Wind Hashira. Kokushibo then gets a good enough cut on Sanemi, stating that moving would lead to his entails to spill out. Sanemi proceeds to laugh hysterically, stating: "The smell of my blood intoxicates demons! Even for marechi, it's rare blood!! Taste it in full!!" Sanemi uses his abnormal body and breathing skills to control his muscles to coagulate the blood and stop the bleeding as much as possible. After some struggling and Kokushibo showing glee at the opportunity at fighting such a skilled Hashira and about to deal a significant blow until Gyomei the Stone Hashira arrives.

Kokushibo is now unable to retain himself. Openly stating that: "Wonderful. The perfect physical form developed to the utmost. How long has it been since I've set my eyes on such a great swordsman, perhaps 300 years?" Following an intense back and forth battle, Gyomei reveals his trump card he meant to save for Muzan, his Demon Slaying Mark. Kokushibo then confirms that he too has the Demon Slaying Mark. Almost showing sympathy towards Gyomei, he reveals the caveat of the Demon Slaying Mark: "Without exception, the marked ones die before they reach the age of 25." Stating that the mark reduces life expectancy and that Gyomei should be dead by the end of the night since he's 27 years old. After an existential discussion of what it means to be a Demon Slayer and a human, Kokushibo offers Gyomei an opportunity to become a demon so he can survive and continue to hone his craft the same way he has. Gyomei outright rejects the offer and states that Kokushibo stated one falsehood: "I suspect there was an exception. There was one who had the mark, lived past 25 and is still alive." Kokushibo then appears to be somewhat shook, but the two continue their intense battle. After Sanemi manages to stitch his wound, he rejoins the fray as the two now face Kokushibo in a 2 on 1 battle. The three of them continue their battle until suddenly, Sanemi and Gyomei are cut by what appears to be an elongated sword with 3 blades extending out the sides(2 on one side and 1 on the other). Kokushibo now appears to be completely serious in his encounter, angrily stating: "You merely shredded my kimono, not even enough to kill an infant. Once I dispatch you two, the rest will proceed easily."

During the meantime, Muichiro manages to free himself from the pillar. And with the little life left in him, helps Genya by reattaching his body and feeding him a piece of Kokushibo's hair. Genya then feels power he's never felt before, but more importantly appears to directly hear Muzan's words meant for Kokushibo. With Muzan stating: "Kokushibo, how many harisha have you defeated? Don't let them come here yet." Indicating that Muzan is likely still incapacitated by the initial explosion and the Blood Demon Art Tamayo performed on him.

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 Gyomei 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 Gyomei 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,Kokushibo himself has continued to develop this breath style and has created multiple techniques due to having lived for centuries.
 * 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 Moongazing (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 Sanemi Shinazugawa.
 * 7) Seventh Form: Mirror of Misfortune - Moonlit (ノ ・え) - Kokushibo swings his sword in a powerful frontal slash that then creates a multi directional frontal assault, powerful enough to create several deep gauges in the ground and push back at least two pillars.
 * 8) Eighth Form: Moon-Dragon Ringtail (ノ) - Kokushibo Uses a direct frontal attack that creates single gigantic slash.
 * 9) Ninth Form: Waning Moonswaths (ノ り・) - Kokushibo creates a seemingly endless stream of slashes capable of cutting down his intended target from long range.
 * 10) Tenth Form: Drilling Slashes - Moon Through Bamboo Leaves (ノ ・) - Kukushibo creates a triple layered slash twister capable of mowing down his targets into three clean pieces.
 * 11) Eleventh Form:
 * 12) Twelfth Form:
 * 13) Thirteenth Form:
 * 14) Fourteenth Form: Catastrophe Tenman Crescent Moon - Kokushibo swings his sword and creates a chaotic vortex of powerful omni-directional slashes that destroys whatever is caught up within its attack radius.
 * 15) Fifteenth Form:
 * 16) Sixteenth Form: Moonbow - Half Moon -Kokushibo brings down a downward slash that then produces a powerful sixfold slash crashing down on his opponents,the attack itself is powerful enough to create several miniature craters where the slashes have landed.

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 and further increase its size.

See Through World - Similar to Tanjiro,Kokushibo has also obtained the ability to see the "see through world" that allows him to see the muscles,blood flow and joint movement of his opponents allowing him to accurately predict and anticipate the movements and attacks of his opponents.

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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