Kokushibo is a major supporting antagonist of Demon Slayer: Kimetsu no Yaiba. He is a demon affiliated with the Twelve Kizuki, holding the highest position, Upper Rank One. Nearly five centuries ago during the Sengoku Era, Kokushibo was a human by the name of Michikatsu Tsugikuni , a former Demon Slayer, and the older twin brother of Yoriichi Tsugikuni, the strongest Demon Slayer that had ever existed. Kokushibo is also the ancestor of Muichiro Tokito and Yuichiro Tokito and the one responsible for turning Zenitsu Agatsuma's senior, Kaigaku, into a demon and joining the Twelve Kizuki. Kokushibo is a very tall man of muscular stature and pale skin complexion. He possesses long, spiky black hair with red tips that he keeps in a ponytail. His most notable feature is the three sets of eyes on his face with yellow irises and red flesh sclera that have black straight lines diverging from each iris. His middle set of eyes has the daiji of "Upper Rank" and "One" engraved in them while his other sets of eyes feature black pupils and a cracked pattern in the irises. Kokushibo also has red markings resembling flames on the top left side of his forehead and the bottom right of his chin. His appearance is described to be profoundly majestic and dignified by Muichiro Tokito. Kokushibo wears a purple-and-black hexagonal patterned nagagi kimono and black umanori-styled hakama pants tied with a white uwa-obi. He carries a fleshy katana at his waist that has eyes in the space between the tsuka ito wrapping of the handle of his sword. The tsuba and blade are shown to have eyes and veins. The scabbard is also fleshy in appearance. As a human, Kokushibo's look was almost identical to his demon form, except for the amount of eyes, his eminent eyebrows, and less light skin. His eyes had dim purple irises and normal, white sclera as a human. As a child, he was normally seen wearing a white kimono as opposed to the purple-and-black patterned one he's seen wearing as an adult. After being beheaded by Sanemi Shinazugawa and Gyomei Himejima, Kokushibo evolves into a more grotesque and monstrous form, with protruding fangs and mandibles, large white horns on the front and and back of his head, pointed nails, red outgrowths on his face, his pupils looking at different directions, shorter and messier hair, several thin tubes poking out of his body, and numerous black and red scorpion tail-like appendages haphazardly sprouting all across his entire body. Kokushibo is reserved, silent, and aloof, maintaining an aura of unnerving tranquility and mystery that compliments his position as Upper Rank One. He rarely speaks; when he does, he talks in a slow and emphatic manner that gives more gravitas and authority to his words. He is an adamant rule-follower, punctual, and he shows a great deal of respect to the hierarchy of the Twelve Kizuki, as seen at the Upper Rank gathering. Kokushibo is shown to be humble as well, and he did not hesitate to admit his failure or complain of any difficulty. He displays a very strong sense of loyalty towards Muzan Kibutsuji, with most of his actions being done with the sole purpose of fulfilling Muzan's goals. Despite exhibiting outward reservation, dignity, and humility, there is a cold and unforgiving side to his personality; his words can be extremely harsh, especially when reprimanding individuals. His threats come off as nothing short of cruel and overwhelmingly serious, and he demands absolute obedience. Kokushibo is shown to be genuinely delighted when the opponents he faced challenged him; such as helping dress Muichiro Tokito's amputated hand over his talented swordsmanship, praising Gyomei Himejima for his unparalleled human abilities and sparing Akaza because he enjoyed the challenge of battling him. However, due to this, he also felt irritation after he realized Akaza had died and he pities Gyomei for his impending death from unlocking his Demon Slayer Marks. He possesses a complex relationship with his human past. He shows something of warmth towards his descendant, Muichiro, commending his skills and resolve, and being moved to convert Muichiro into a demon to spare his life. When their battle comes to an end Kokushibo displays his regret at having struck down his descendants. At the same time, he refers to Muichiro and his abilities as the natural result of his "cells" being passed down. In his duel against Gyomei, the demon's banter suggests a fear of death. When Gyomei's marks appear, Kokushibo pre-emptively bemoans the loss of a talented fighter and he urged him to become a demon to continue honing his skills. He seemed surprised when Gyomei vehemently rejected his offer and called his mentality pathetic. It is revealed that as a human, Kokushibo harbored an immense sense of envy towards his younger twin brother, for his natural talent and incredible abilities. These feelings of jealousy and contempt only became stronger upon seeing his brother become a peerless warrior of unmatched caliber among even the Demon Slayer Corps, with none of the Hashira coming close to his level of strength, and this envy peaks when he discovers that Yoriichi was still alive and in old age, despite the curse of the Demon Slayer Marks supposedly killing him before he turned 25. However, despite this immense jealousy, spite, and the outright hatred he harbors for Yoriichi, he still deeply cares for his brother, as seen when he was touched by Yoriichi treasuring the handmade flute he had made for him as children; Kokushibo sheds tears over his brother's death, and even goes as far as keeping the flute itself for the following centuries as a memento. Likely a result of his inferiority complex and his immense desire for strength, Kokushibo fears the prospect of suffering defeat. So much so, he became increasingly more aggressive and agitated when being pushed further into a corner, causing him to become more desperate in his attacks and the use of his demon power. During his battle with the Hashira, he came to realize the steep price he had paid for the strength he desires so much; becoming an ugly monster far from his idealized dream of becoming the strongest samurai just like his brother, symbolizing how much his deep ambitions and resentment had warped him for the worst. In his last moments, he greatly laments and regrets his choices in his life and began to openly question himself if the path he walked was truly the right one. Kokushibo dies filled of regret, sorrow, and rage for not being able to accomplish anything that he had wanted, angrily asking his deceased brother why he was even born as he faded into ash.