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 himself realized that this revelation would soon cause the brothers' roles within the household to be reversed. Michikatsu would no longer considered to be the successor of the family, 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. This event fueled Michikatsu's desire to master the ways of the sword and would stoke the brewing emotions of jealousy towards his brother.

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 hated 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 would meet for the last time. Michikatsu was unnerved by Yoriichi's mourning of his decision, but was nonetheless frightened upon seeing him ready to attack. When Yoriichi struck, Kokushibo was too slow to react and was nearly beheaded. Waiting for the second strike, he realized Yoriichi had died of old age and angrily slashed his body for the feeling of inferiority he attained throughout his life.

Despite the monstrous jealousy he harbored towards Yoriichi, Michikatsu still held a complex, deeply buried love for his younger brother. This is heavily implied by the tears shed by him after realizing that the flute he had gifted his sibling had remained with Yorichii after all this time. In turn, Michikatsu could not part with the flute after the latter's death, and kept it on his person for hundreds of years thereafter until his own demise. Eventually, in his final moments, he acknowledges his brother was beyond him in all ways and asked him for why he was even born.

Muichiro Tokito
When 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 Kokushibo's. 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 displays his remorse at having struck down his descendants.

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, 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.

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 weakling who disrespects Kizuki who are of a higher ranking than him. This is shown the first time when he scolds him for attempting to start a fight with Doma, and later on when he calls him "very weak" for refusing to keep fighting 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.