Kokushibo/Relationships

Yoriichi Tsugikuni
Yoriichi was the identical younger twin of Kokushibo — Michikatsu in this case since they mainly interacted when Kokushibo was still a human — and during their childhood, Michikatsu always looked at Yoriichi as a sort of "inferior being" to him. Since the twins were born, Yoriichi was ostracized from his brother since he was born with a grotesque mark, whereas Michikatsu was chosen to be the successor of the family. He would pity Yoriichi for his childish behavior since he would cling to their mother, however he would still try to get along with him and spend time together with by playing games together, However as a result of this and due to him being the designated heir to the family, their father would harshly beat him and tell him to just forget about Yoriichi and stop spending time with him. Despite this however, Michikatsu would still persist, and make Yoriichi a handmade flute despite his father's beating and told him to blow on it whenever he would be in trouble and he'd come right away to protect him, smiling at him with his bruised face as he happily reassured his younger brother.

However, when Yoriichi began to display his advanced abilities as a swordsman their roles within the household had flipped. Michikatsu was no longer considered to be the successor of the family, and his brother would take his place in becoming a swordsman. This event fueled Michikatsu's desire to master the ways of the sword like his brother, and bring emotions which would influence some of his choices later in life.

When their mother died unexpectedly, Yoriichi informed Michikatsu that he would be leaving home and as he said goodbye, Yoriichi mentions the flute given to him and he smiles while holding it tight. During this time Michikatsu has negative thoughts referring to his smile as "disturbing" and referring to the flute as a "piece of junk" despite being to precious to Yoriichi. After reading his mother's diary, Michikatsu discovers that Yoriichi was not clinging to his mother, but was in fact supporting her since he knew she was ill. This realization infuriates Michikatsu with genuine hatred and jealousy towards Yoriichi, 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 become 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 those who are of a higher Upper Kizuki 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 jealously towards him after learning that Akaza was almost able to regenerate his head before Kokushibo despite him being a lower rank than him.