Kokushibo/Relationships

Yoriichi Tsugikuni
Yoriichi was the identical younger twin of Kokushibo — Michikatsu in this case since this is the name that Yoriichi knew him as. During their childhood, Michikatsu always looked at Yoriichi as inferior to him. When the twins were born, Yoriichi was ostracized from his brother because of the grotesque mark he bore, whereas Michikatsu was chosen to be the successor of the family. He pitied Yoriichi for what he perceived as childish behavior, spurred on by how Yoriichi would cling to their mother. Despite this, Michikatsu went out of his way to talk to and interact with his younger brother, even risking beatings from their father who forbade interactions between the two. On one occasion, Michikatsu snuck in and gave Yoriichi a handmade flute. He told him to blow on it whenever he was in trouble, stating that he would come right away to protect his younger brother.

When Yoriichi was revealed to have a prodigious, natural talent for swordsmanship, Michikatsu was shown to have entered into a deep sense of depression upon realizing what would happen to him once their father learns of his younger brother superior talent. Michikatsu thought that he would be the one put into his brother's small, isolated room and sent to the temple to become a monk once he reaches adulthood while Yoriichi would take his place as the family's heir.

When their mother died unexpectedly, Yoriichi informed Michikatsu that he would be leaving home. As he said his goodbyes, Yoriichi brought out the flute Michikatsu had given to him. He smiled, holding it close, telling Michikatsu that it was precious and that he thought of it as him. Michikatsu was disturbed by his brother's smile and confused about his brother's cherishing of what he considered to be a "piece of junk". After reading their mother's diary, Michikatsu discovered that Yoriichi was not clinging to his mother but was in fact supporting her, having known about how her illness weakened her. This realization infuriates Michikatsu, a deep hatred and jealousy towards Yoriichi taking root within him, to the point where he wishes Yoriichi had never been born and mentally begs for him to die. This hatred only grew when he was saved from a demon by Yoriichi himself, who had become more powerful. After abandoning his family, he joined the Demon Slayer Corps to fight alongside his brother and become stronger. Michikatsu himself tried to learn Yoriichi's Sun Breathing but was unable to, instead using a derivation that became known as Moon Breathing. Michikatsu then transforms into a demon through Muzan after lamenting his inability to ever surpass Yoriichi in power.

Sixty years later, the two met for the last time. Upon seeing what his brother became, Yoriichi shed tears, heartbroken by his decision to transform. On the contrary, Kokushibo looked at Yoriichi with disgust, infuriated that he was able to overcome the curse of all marked Demon Slayers and had lived far past the age of 25. The two dueled for the last time. Yoriichi told his brother when he was to strike. Kokushibo grabbed his blade in anticipation, yet was quickly outmatched, having been slashed across the neck before he could even react. Kokushibo's disgust turned into fear, and then anger as he saw that despite his old age, Yoriichi was still the capable and powerful warrior as he was in his prime.

Kokushibo awaited the second strike, despite knowing it would mean his demise. Yet the strike never came, as Yoriichi soon passed from his age while still standing. This had truly infuriated Kokushibo, seeing that his continuously succeeding brother had transcended nearly everything and died of old age rather than being defeated in battle. Kokushibo angrily slashed his brother's body out of the feeling of inferiority he attained from him throughout his entire life.

Yet despite his chronic envy, Kokushibo could never truly hate his brother. Upon striking Yoriichi's corpse, he realized that he had inadvertently broken the flute he had gifted to him as a child. Seeing how he still thought highly of him even after his transition into a Demon, Kokushibo could not help but shed a tear for Yoriichi. Kokushibo could not part with the flute after the latter's death, and kept it for hundreds of years thereafter until his own demise.

In his final moments, he acknowledged that he only ever wanted to be like his brother and continuously asked him for why he was even born as he disintegrated.

Muichiro Tokito
When the two swordsmen first meet, Kokushibo immediately remarks that Muichiro seems familiar to him. After telling Kokushibo his name, Muichiro is stunned when he is told that he is a descendant of his. The aura given off by Kokushibo is intimidating to the point that Muichiro is left shaking and unable to move. However, he is able to overcome his fear and attack Kokushibo but is easily subdued. Kokushibo openly says that Muichiro has developed swordsmanship for someone his age and is impressed to the point where he will convert him into a Demon to serve Muzan.

As the battle between Kokushibo and the Demon Slayers comes to an end, Muichiro is able to come to a close proximity to him and is able to pierce him with his transformed red katana, temporarily immobilizing him. However, Kokushibo continues to strike and fatally wounds Muichiro. When their battle comes to an end Kokushibo regretted killing his descendants.

Genya Shinazugawa
Kokushibo first met Genya in the aftermath of when he defeated Muichiro Tokito. During a short exchange where he attempts to persuade the Mist Hashira into becoming a demon, Genya snuck behind a pillar and tried to fire his shotgun, only for the pellets to miss and for Kokushibo himself to suddenly appear behind him. The demonic swordsman made quick work of the Demon Slayer, slicing him apart so quickly, Genya had no time to react. Surprised that he was still alive despite being in several pieces, Kokushibo mocks him for being an "imitation demon" and prepared to kill him, but was interrupted by the arrival of his older brother, the Wind Hashira.

Genya was able to heal after consuming a torn lock of Kokushibo's hair, due to its extremely high concentration of Muzan's blood. He spotted a broken-off piece of Kokushibo's sword and realized it is made of his flesh, allowing to gain a portion of his power. Though Genya began to lose faith in himself, believing Kokushibo would simply attack him again and perhaps kill him, he pushes on and swallows the broken blade.

Kokushibo was surprised when Genya was able to land an attack on him, before realizing he had consumed a part of him and gained his strength. Genya was pivotal in Kokushibo's defeat, using vampiric trees to suck his blood and prevent him from using his Blood Demon Art. Though Kokushibo unleashes a devastating attack that cuts Genya in half, the Demon Slayer held on and pinned him down with his Blood Demon Art, allowing the others to strike Kokushibo with their Bright Red Nichirin weapons, eventually leading to his death. However, Genya succumbed to his wounds and died shortly in the aftermath of the fight.

Sanemi Shinazugawa
Sanemi faced Kokushibo as the latter was about to decapitate his younger brother. Enraged at the Upper Rank's attempt on Genya's life, Sanemi faced Kokushibo head-on. He was initially excited at the prospect of battling Upper Rank One but quickly realized his opponent was on another level of strength, skill, and speed. In their clash, Sanemi sustains numerous cuts to his body but his blood is able to lower Kokushibo's alertness, though the demon is able to shrug off the effects and regain focus. Sanemi was on the verge of dying to Kokushibo's attacks but was rescued by Gyomei Himejima's intervention. After awakening his Demon Slayer Marks, Sanemi fought with Gyomei to push back Kokushibo, only for the demon to enhance his weapon in retaliation. Sanemi is overwhelmed by the sheer number and ferocity of Kokushibo's Moon Breathing forms, losing two fingers from a tiny lapse in focus, and he was only rescued by the sacrifice of Muichiro Tokito and his brother's Blood Demon Art. After being trapped by the Demon Slayers, Kokushibo mocked them all for being unnatural humans; in particular, he insulted Sanemi's inhuman nature for surviving such devastating injuries without losing strength.

Ultimately, it was Sanemi and Gyomei who succeed in decapitating Kokushibo after turning their Nichirin weapons bright red. Though Kokushibo grows back a new head and prepares to heal, Sanemi's blade causes Kokushibo to see his reflection, and realize the monster he had become in his pursuit of power. In the end, Sanemi and Gyomei work together to slice apart Kokushibo's body a second time, this time leading to his death.

Gyomei Himejima
When he first met the Stone Hashira, Kokushibo marvelled at his physical abilities, claiming it had been 300 years since he last saw someone of Gyomei's caliber. After an initial clash, Gyomei was forced to use his Demon Slayer Marks to even the playing field. Kokushibo pitied him for wasting his life and power just to die, noting that past the age of 25, he was doomed to perish soon. Gyomei rejects this statement and unexpectedly taunts Kokushibo by claiming someone marked and above 25 years of age had lived in the past. Kokushibo then attacks him with renewed ferocity after remembering his hated brother.

Remembering Yoriichi's words that a generation will surpass their own already powerful era of Demon Slayers, Kokushibo became scornful of the Hashiras fighting. In particular, he mocked Gyomei as unnatural, for surpassing the limits of humans and attaining a growth in power like demons. Eventually, Gyomei and Sanemi are able to behead Kokushibo. Despite regrowing a new head, Kokushibo's healing stopped from fighting his humanity and Genya's Blood Demon Art, allowing the Stone Hashira to decapitate once more, this time, leading to his death.

Muzan Kibutsuji
Kokushibo first met Muzan when he had recently learned that his Demon Slayer Marks would cause him to die at a young age. Muzan, wanting a demon who used a Breathing Style, offered him a chance to live and become stronger forever. Taking the offer, he transformed into a demon, took up a new name and eventually attained the rank of Upper One among his Twelve Kizuki. Kokushibo was loyal to Muzan, acting as enforcer to the Twelve Kizuki, as shown when he reprimanded Akaza for attacking Doma.

In return, Muzan sees him as a business partner, possibly due to his fanatic loyalty to him, being his strongest Demon and because the two were jointly responsible for almost exterminating Sun Breathing. Despite this, Muzan would nonetheless admonish him along with the rest of the Upper Ranks, for their inability to find the Blue Spider Lily and not killing the Ubuyashikis. Kokushibo didn't retort his statement and simply claimed the Ubuyashikis have hidden themselves from view.

Akaza
There is hostility between Kokushibo and Akaza. He doesn't get along with Akaza, since he sees him as a disgruntled individual who disrespects the Kizuki who are of a higher ranking than him. Akaza returns this negative sentiment, shown when he admonishes Upper Three for attacking the higher ranked Doma and was then threatened with death by the enraged demon, and later on when he denounced him "exceedingly weak" for refusing to continue his battle against Giyu Tomioka and Tanjiro Kamado. However, despite the fact that he does not get along with Akaza, Kokushibo later shows acknowledgement towards him after learning that Akaza was almost able to regenerate his head, which inspires Kokushibo to attempt the same.

In the past, Kokushibo was once challenged by Akaza for his position of Upper Rank One, though Upper Three was promptly defeated by the demonic swordsman. Despite Kokushibo usually absorbing the demons he defeats during his battles, he did not do so to Akaza and let him go due to finding the challenge entertaining. When Akaza had threatened to kill him during, Kokushibo did not react with further hostility and even encouraged the demon to find a way to do so.