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, the brothers' roles within the household were reversed. Michikatsu was no longer considered to be the successor of the family, taking up his brother's small, isolated room while Yoriichi took 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.

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.

Yuichiro Tokito
Kokushibo did not have any interaction with Yuichiro since he had died years before Muichiro became a swordsman. However, Yuichiro is a descendant of Kokushibo's immediate family.

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.

Muzan Kibutsuji
Not much is known about Kokushibo's perspective towards Muzan, He was willing to become a demon on his own accord because he believed he wouldn't live long after he had became marked. However it can be assumed that Kokushibo does hold Muzan at a regard as he calls him "Lord" as well as "his lordship". In return, Muzan sees him as a business partner.

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.