Warning: This contains spoilers for the Demon Slayer manga not yet covered in the anime.

Kokushibo is a relatively recent addition to the Demon Slayer lineup and while a lot of information is known about him, he still remains an intriguing figure. BeAt the time of his introduction he was one of the oldest Demons that the audience was aware of besides Muzan himself, he has a long and confusing history full of conflicts, battles, and relationships. Frequently he was victorious, rarely did he lose, but he was never quite able to shake the worries and insecurities of his human life, even after 400 years. Ultimately his all too human failings caused his downfall, just as they caused his descent into the demon race.

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Demon Slayer follows young Tanjiro and his sister Nezuko as he climbs his way up through the ranks of the Demon Slayer Corps. He does so by challenging increasingly powerful demons in an ultimate bid to defeat the leader of the demons so that he can return his sister to her humanity.

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As a Human

Kokushibo was a human during the Sengoku era, and was the older brother of Yoriichi Tsugikuni. As a human, he was a powerful swordsman and respected member of the Demon Slayers corps. However, he was never able to step out of the shadow of his twin, the inventor of Sun Breathing - from which all other breathing styles are derived. Out of a sense of inferiority, he invented his own breathing technique, Moon Breathing, though it too was derived from Sun Breathing and therefore did little to assuage his jealousy. This struggle plagued him until the moment of his death.

He studied under his younger brother and in this way was able to unlock his Demon Slayer Mark, which unlike his brother’s, was both on his forehead and chin. When he unlocked this mark it came with mixed reactions; he had attained greater skills, but also acquired the curse that would take his life at the age of 25. He was so afraid of death that he allowed himself to become a Demon and therefore immortal. This makes him one of the few known Demon Slayers to defect and turn to become a Demon. In doing so he abandoned a wife and two children, though unlike many new demons he did not slaughter the vestiges of his human family.

As a Demon

Once Kokushibo became a demon he underwent a radical physical transformation. He acquired two additional sets of eyes and a far more dramatic demeanor. He lived for around 480 years as a demon, successfully thwarting the Curse of the Demon Slayer Mark. Over these years he gained and handily held the rank of Upper One of the Twelve Kizuki. It is well known that he defeated multiple challengers for the role, though left only Akaza alive due to the challenge that he posed. For the rest of the defeated would-be usurpers he utilized one of his unique skills to absorb, it is also believed that this may have been his more efficient manner of consuming humans.

As a demon, he also gained tremendous power that did allow him to eventually defeat his brother, though he did not gain the anticipated satisfaction from it. Instead, his joy was overshadowed by the realization that his brother had managed to evade the curse and live well beyond 25 years, introducing doubt that he had made the right choice in becoming a demon. Again the viewers aren’t treated to the fruition of this doubt until the moment of his death.

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Additionally, as a Demon Kokushibo is able to augment his Breathing Style with his Blood Demon Art, making Moon Breathing into a far more powerful attack: Crescent Moon Blades. It is unclear exactly how many of the forms of Moon Breathing Kokushibo was able to develop as a human and how many were developed by him as a demon, but each form is certainly supplemented by his Blood Demon Art.

Despite becoming a demon Kokushibo chose not to give up his Nichirin blade, instead modifying it with his own blood and flesh to create the new and improved Flesh Katana. Since it is modified with his body it is arguably sharper and more durable than the average Nichirin Blade. He created for it an equally monstrous Fleshy scabbard.

In Death

When he was finally defeated it was at the hands of one of the Hashira, Gyomei, with the help of one of his own descendants, Muichiro, and a few others. In the moment of his death, Kokushibo had a final moment of clarity. He realized that his quest for strength, inspired by jealousy for his brother, had led him down a monstrous path. Instead of becoming the ideal samurai as he had once dreamed, he had forsaken everything good about himself and descended into something that he himself recognized as an ugly monster.

He even lamented the termination of his own bloodline at his hands. Though in death he denied that his original motivation in becoming a demon had been fear of death. Instead, his obsession with his brother is only emphasized in his final words, over 400 years after the death of Yoriichi.”Yoriichi… I just wanted to be you.”

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