Kokushibo/Synopsis

Human Life
Born in the Sengoku Period, Kokushibo's birth name was Michikatsu Tsugikuni. When he and his brother Yoriichi were born, twins were considered a bad omen, and when their father saw that Yoriichi had a strange birthmark on his forehead, he declared that he would kill Yoriichi. However, their mother Akeno Tsugikuni was outraged at his announcement, and an agreement was made that instead of being killed, Yoriichi would be sent to the temple and become a priest upon turning ten. Yoriichi and Michikatsu grew up very differently, Yoriichi being confined to a three-tatami room separate from the rest of the house and kept away from his brother. Michikatsu, who snuck around behind their father's back to see his brother, pitied Yoriichi, who was thought to be deaf until he was seven and clung to his mother's side whenever Michikatsu saw her. At some point, he gave Yoriichi a flute he made as a gift. Around when Michikatsu was seven, he was practicing sword swings while his brother watched. It was then that Yoriichi spoke for the first time, shocking Michikatsu; however, Yoriichi's declaration of his goal to become a samurai like his brother was dismissed, as he would be sent to the temple when he was ten. Nonetheless, Yoriichi continued to watch Michikatsu's training, and one day, one of their father's men offered to give Yoriichi some instruction. Though he only showed Yoriichi a basic stance, the boy was able to land four blows on the man. This incident bewildered Michikatsu, who had never before landed a blow, and he begged his brother to teach him his secret. To Michikatsu's disappointment, Yoriichi only expressed his distaste for combat and stated that he no longer wanted to become a samurai. Once, however, Yoriichi mentioned that before he took a stance, he breathed deeply, examining his bone alignment, muscle contraction, and blood flow, implying an early form of the Sun Breathing style that Yoriichi would later adopt. This conversation caused Michikatsu to realize that for Yoriichi, the bodies of living things were transparent, causing him to pursue mastery of the blade with renewed intensity. As Michikatsu studied the sword, he felt envious of his brother, who he felt was squandering his natural physical talent. He also feared that, now that their father likely knew about Yoriichi's immense potential, their roles would be swapped and he would be the one sent to the temple in Yoriichi's place. Yoriichi recognized this as well; the night that their mother finally succumbed to her illness, he came to Michikatsu's room at three in the morning to tell his brother the news and bid goodbye to Michikatsu, as he planned to leave for the temple before their father could make Yoriichi the family's new successor. Yoriichi expressed gratitude to his brother for giving him heart and fondly held the flute that Michikatsu had given him earlier. Michikatsu, however, was irritated, wondering how Yoriichi could treasure a flute that couldn't even play in tune, and watched his brother depart while saying nothing. His jealousy of his brother only grew when he read his mother's diary following her death, which revealed that Yoriichi had known everything about their mother's illness from when she would die and the increasing weakness of the left side of her body, and had planned accordingly. From then, Michikatsu resented Yoriichi, who he realized to be not only physically gifted, but a genius as well. He wished for his brother's death and for the next few years, not hearing anything from him, he believed it to be true. Ten years later, Michikatsu became a samurai and married a woman who gave birth to two of his children. One night, a Demon attacked his camp and left him the only survivor, but was quickly killed by Yoriichi who came to his rescue and apologized for not being able to save his subordinates. Seeing the object of his hatred again and twisted by jealousy, Michikatsu abandoned his family and became a Demon Slayer to train with Yoriichi. He saw his brother attempt to teach the other Demon Slayers his own Breathing technique without success. Regardless, he altered them to suit their own strength, creating the first Breathing Techniques. During that time, he trained alongside a Wind Breathing user and polished his sword skills together with them. Eventually, Michikatsu awakened a Demon Slayer Mark and developed a Breathing Style derived from the Sun Breathing known as the Moon Breathing. Despite this accomplishment, it left a bitter taste as it was still not as powerful as Yoriichi's own Sun Breathing.

As a Demon Slayer, Michikatsu was often prideful in the uniqueness of their generation, but was irritated by his brother's optimism when he brought up the fact that the pair had no successors in line for their unique Breathing Styles, the Sun Breathing and Moon Breathing.

Demon Life
Frustrated from being unable to surpass his brother, as well as fearing the fated death of those who awakened their Demon Slayer Marks, Michikatsu was persuaded by Muzan Kibutsuji and became a Demon, taking the name Kokushibo.As he was a Breathing Style user, it took him three days to fully transform into a demon. Eventually, he became Upper Rank One of the Twelve Kizuki, remaining at that rank for hundreds of years, while his original name was lost to history.

Only soon after becoming a demon, he killed the Oyakata that led his era of Demon Slayers and brought his head to Muzan.His young son was left to lead the Demon Slayer Corps afterwards.

Shortly after he became a demon, his leader Muzan encounter another Demon Slayer, Yoriichi himself. Since Muzan already had him as a subordinate, he nonchalantly tried to kill his twin, unable to sense any anger or aura of power in him. Instead, Yoriichi was able to completely overwhelm the Demon King and nearly killed him. Muzan barely escaped with his life and enlisted Kokushibo's aid to wipe out Sun Breathing. As Yoriichi was banished from the Corps, Muzan and Kokushibo were able to kill all the swordsman that used the Breathing Style. Sixty years after becoming a Demon, he came face to face with Yoriichi for the last time. Yoriichi was devastated by his decision to transform, whereas Kokushibo became disgusted seeing his younger twin now aged and decrepit, as well as infuriated upon realizing he defied the fate of all marked Demon Slayers by being far older than 25 years of age. Beyond this, Kokushibo wasn't as angry seeing him as he expected. Seeing Yoriichi cry shocked Kokushibo, as it was the first time he had shown emotion in his presence, confusing him. As Yoriichi was still a Demon Slayer and wanting to rid his human past, he knew he had to kill his younger brother. Despite his old age and apparent blindness, Yoriichi's fighting stance in preparation to strike frightened Kokushibo, who knew then his immense skill and power was still presence. Despite gripping his blade in anticipation, Yoriichi had already struck and sliced across his neck. Kokushibo became angry seeing his brother's strength had not weakened in the slightest and waited for a return strike, despite knowing it would certainly kill him.

Yoriichi never had the chance to cut Kokushibo's head entirely off, dying due to old age while standing up. At that moment, Kokushibo became truly furious, seeing that his continually excelling brother had appeared before him having transcended everything and for dying of old age instead of losing a battle, as well knowing that even Muzan couldn't lay a scratch on Yoriichi, his one chance at a fair battle is now gone and his brother remains undefeated forever. Enraged by the feeling of inferiority, he slashed Yoriichi's body in half, but inadvertently broke the flute he gave him as a child. Seeing that he still thought highly of him even after his transformation, Kokushibo couldn't help but shed a tear. Unable to truly sever his bond with his brother, he decided to keep the flute for himself.

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-devouring Demons to enhance his own strength. It is implied that this Demon Slayer's ability wasn't as adept as Genya's given that Kokushibo slew him just by cutting his torso into two.

Entertainment District Arc
Kokushibo was summoned to a meeting of the Upper Ranks, 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 Muzan had arrived. Muzan revealed the death of Gyutaro, berating Kokushibo and his fellow Upper Ranks 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 Ranks 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 Rank, which Kokushibo wished him luck before departing.

Infinity Castle Arc
During a meeting of the Hashira reporting the appearance of strange marks, Amane Ubuyashiki recalls that these marks were common among a select few users of the original Breathing Style, Kokushibo having been one of those few.

Kaigaku remembers his encounter with Kokushibo while engaging Zenitsu. Then, Kokushibo overwhelmed him and he quickly surrendered. But he decided to give Kaigaku a second chance and offered him a large amount of Muzan's blood, then hoping Muzan would accept him, he could take a place among the Upper Moons. Kokushibo told the demon slayer to be grateful for his gift but threatened to behead him should he spill a drop.

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, and reprimanded him for how weak he is. Simultaneously, he reaches for his katana, and instantly slices the surrounding stone pillar to pieces in a single stroke.

Shortly after, Muichiro Tokito is transported to the same room as Kokushibo. Kokushibo then comments: "You came... Demon Slayer. For some reason... 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 Mist Breathing, 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, Muichiro is effortlessly pinned against the pillars when Kokushibo impales him with the Muichiro's own sword. 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 effortlessly 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 demons, huh?" However, he realises 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. After Genya apologises for everything he's done to Sanemi, Kokushibo states that: "Hmm... brothers and Demon Slayers, that brings back memories...". Sanemi and Kokushibo then begin their intense duel, with Kokushibo being slightly taken aback at the skill of the Wind Hashira. However, he retains his composure and stated that he couldn't survive if his senses weren't honed from long experience. This goes on for sometime, with Kokushibo even stating that long ago, in the Sengoku era, 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 entrails to spill out. Sanemi proceeds to laugh hysterically, boasting the fact that he had marechi blood. 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, he was about to deal a significant blow until Gyomei, the Stone Hashira arrives.

Kokushibo was unable to retain himself. Openly stating that he admired Gyomei's peek physical form. 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 shaken, 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. Kokushibo notes with surprise the ease with which they are able to cooperate despite being users of two different breaths. The three of them continue their battle until suddenly, Sanemi and Gyomei are cut by what appears to be an elongated sword with three blades extending from the sides, two from one side and one 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."

In the meantime, Muichiro manages to free himself from the pillar. With the little life left in him, he helps Genya by re-attaching his body and feeding him a clump of Kokushibo's hair. Genya then feels power he's never felt before, but more importantly appears to directly hear Muzan's voice, asking Kokushibo how many Hashira he has defeated. indicating that Muzan is likely still incapacitated by the initial explosion and the Blood Demon Art Tamayo performed on him.

The battle then begins to pick up, Kokushibo's attacks increasing with both intensity and frequency. He is able to sever two of Sanemi's fingers, and Gyomei and Sanemi are both reduced to putting all of their efforts into simply avoiding his attacks. Right as it seems that Sanemi is about to lose, though, Muichiro rejoins the battle, saving the other Hashira just in time. Genya, who is hiding from the battle behind a wall, recalls a conversation he had with Tanjiro about being the weakest person in a battle, and, motivated by this, swallows a broken piece of Kokoshibo's blade in order to gain its abilities. The advantage seems to again shift to Kokushibo, but in an unexpected turn of events, Gyomei is able to sever the Upper Rank's right arm while Muichiro stabs him through the torso, immobilizing him while Genya unleashes a barrage of bullets on the demon using his gun, which he altered using his own cells in a similar manner to Kokushibo's sword. The bullets embedded into the Demon's flesh then erupt into "trees," completely trapping him as Gyomei and Sanemi rush in to finish him off. The sensation disturbs Kokushibo, who is reminded of an encounter from 400 years ago in which he met his younger brother, Yoriichi, after not having seen him for 60 years. In the encounter, Yoriichi was able to land a hit on his brother's neck, which no one had ever done before; however, Yoriichi passed away on his feet before he was able to land the next blow.Enraged by the thought of being defeated, Kokushibo sprouts multiple blades from his body and mortally wounds Muichiro and Genya and severely injures Sanemi and Gyomei. Still, the four charge onwards, undeterred. As Muichiro's blade turns red, seemingly seering Kokushibo's organs, Genya unleashes another Blood Demon Art, rooting another tree in Kokushibo's back while Gyomei lands a blow on Kokushibo's head with his flail and Sanemi rushes in with his sword. Their weapons all begin to turn red, and, struck with thoughts of his brother, Kokushibo marvel at how the Demon Slayers who came after them have risen up to surpass them. However, wonder soon gives way to rage as Kokushibo's head is cut off, and, to the horror of the four Demon Slayers, Kokushibo regrows his head. Kokushibo begins to celebrate, boasting to himself that now that he has conquered death by beheading, any attacks thrown at him are now rendered meaningless. However, his triumph gives way to disbelief when he sees himself in the reflection of Sanemi's blade. His new form is monstrous, with tusks and spikes protruding from his head, face and back. Disgusted by his new appearance, he recalls his original motivation to be the strongest samurai in the country and wonders if the path he has taken is what he really wanted. While he reflects on his motivations, Gyomei is able to crush his head with his flail, again beheading Kokushibo. The Upper Rank attempts to regenerate, but to his disbelief, his flesh begins to crumble away and he is rendered unable to use his Blood Demon Art as a result of Muichiro's red blade stuck in his body.

As he falls to the ground, disintegrating, he reflects bitterly that he only wanted to be like Yoriichi. He becomes reminded that the one thing he wanted to forgot is the thing he can only remember vividly, Yoriichi's face. While his life flashed before him, he admits that he abandoned everything and in the end, has achieved nothing by killing his only remaining blood relative and betraying his samurai code. In his final moments, he questioned why he was ever born, calling to his brother for an answer, before fading away completely and leaving only his broken flute behind.

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