Why Michikatsu Tsugikuni Betrayed Humanity to Become Kokushibo

Michikatsu Tsugikuni is the human name of the primary antagonist Kokushibo, the Upper Rank One demon within the Twelve Kizuki in the Demon Slayer: Kimetsu no Yaiba series. Born during the Sengoku Period—often referred to as the Golden Age of Demon Slayers—Michikatsu was a high-ranking samurai and the older twin brother of Yoriichi Tsugikuni, the strongest swordsman to ever live. His transformation into a demon was not motivated by a desire for simple bloodlust, but by a profound, centuries-long obsession with surpassing his brother’s unmatched genius.

To understand Michikatsu Tsugikuni is to understand the tragedy of an elite talent living in the shadow of a god-like anomaly. While Michikatsu was a master of his craft, his descent into demonhood remains one of the most complex psychological explorations of envy in modern manga history.

The Dual Destiny of the Tsugikuni Twins

The story of Michikatsu Tsugikuni begins with a perceived curse. Born into a noble samurai family, the arrival of twins was viewed as a bad omen, sparking a potential succession crisis. The disparity between the brothers was immediate: Michikatsu was groomed as the heir, receiving the best education and sword training, while Yoriichi was treated as a social outcast, confined to a small room and expected to be sent to a temple to become a priest.

However, the power dynamic shifted during their childhood. Michikatsu, initially pitying his "deaf" and "weak" younger brother, gave Yoriichi a handmade flute—a gesture of brotherly love that would later become a haunting symbol of his regret. The turning point occurred when Yoriichi, having never held a sword, defeated a trained instructor in four swift blows. It was in this moment that Michikatsu realized his brother was not just gifted; he was a natural force of nature.

Michikatsu’s envy was birthed from this realization. He discovered that Yoriichi possessed a unique biological trait: the ability to see the "Transparent World" (Sukashi no Sekai), allowing him to perceive the muscles, blood flow, and joint movements of living beings. For Michikatsu, who spent every waking hour training to achieve perfection, Yoriichi’s effortless mastery was an insult to the concept of hard work.

The Resurrection of the Samurai and the Birth of Moon Breathing

After their mother’s death, Yoriichi disappeared, leaving Michikatsu to inherit the family name, marry, and have children. For a decade, Michikatsu lived a standard samurai life until a demon attacked his camp. He was saved by Yoriichi, who had become a Demon Slayer. The sheer gap in their power remained immense. Driven by a renewed obsession to reach Yoriichi’s level, Michikatsu abandoned his family and his status to join the Demon Slayer Corps.

During this era, Yoriichi attempted to teach the other slayers his Sun Breathing, the progenitor of all breathing styles. However, no one could replicate it. Michikatsu, despite his immense talent, was also unable to master Sun Breathing. Instead, he developed a derivative style: Moon Breathing.

How does Moon Breathing differ from Sun Breathing?

While Sun Breathing is characterized by its fluidity and solar heat, Moon Breathing reflects Michikatsu’s technical precision and his inability to reach the "source." Moon Breathing creates chaotic, crescent-shaped blades of energy that vary in size and length. It is a style built on the foundation of high-level swordsmanship but augmented by a desperate need for complexity to compensate for the lack of Sun Breathing's pure power.

The Demon Slayer Mark and the Deal with Muzan Kibutsuji

As the brothers led the Demon Slayer Corps, they both awakened the Demon Slayer Mark. While the mark granted superhuman physical capabilities and access to the Transparent World, it came with a fatal price: the "Curse of the Mark," which dictated that anyone who awakened it would die before the age of 25.

Michikatsu faced a terrifying dilemma. He was reaching the peak of his swordsmanship but was running out of time to surpass Yoriichi. His fear of death was not a fear of non-existence, but a fear that he would die without ever proving he was superior to his brother. It was at this moment of vulnerability that Muzan Kibutsuji, the Demon King, approached him.

Muzan offered a simple solution: become a demon, achieve immortality, and you will have centuries to polish your techniques. Driven by a toxic mixture of ambition and dread, Michikatsu accepted. He consumed Muzan’s blood, a process that took three agonizing days due to his strength, and emerged as the demon Kokushibo.

The Anatomy of Upper Rank One: Kokushibo’s Power

As Kokushibo, Michikatsu retained his samurai discipline but evolved into a nightmare of biological efficiency. His appearance is defined by his three pairs of eyes. These six eyes grant him a panoramic field of vision and an enhanced version of the Transparent World, making it nearly impossible to land a surprise attack on him.

The Flesh Katana: Kyogusari

Unlike other demons who use separate weapons, Kokushibo’s sword is a manifestation of his own flesh and blood. Known as the Kyogusari, the blade is covered in eyes and can regenerate instantly if broken. Because the sword is part of his body, he can manipulate its shape, growing extra blades from the sides or extending its length to cover massive areas. This allows his Moon Breathing techniques to reach a scale that no human swordsman could ever achieve.

The 16 Forms of Moon Breathing

Kokushibo’s mastery of Moon Breathing spans centuries, resulting in at least sixteen known forms. His Blood Demon Art enhances these forms by populating the air around his swings with "crescent moon blades"—tiny, unpredictable energy shards that shift in size and trajectory.

First Form: Dark Moon, Evening Palace – A swift, singular horizontal slash that leaves a trail of crescent blades.
Second Form: Pearl Flower Moongazing – Several crescent slashes that defend the user while attacking.
Third Form: Loathsome Moon, Chains – Two massive, swinging slashes that create a storm of smaller blades.
Fifth Form: Moon Spirit Calamitous Eddy – A layered vortex of slashes that destroys everything in a wide radius.
Sixth Form: Perpetual Night, Lonely Moon - Incessant – A rapid-fire barrage of vertical slashes.
Seventh Form: Mirror of Misfortune, Moonlit – An extended reach attack where the sword grows multiple branches.
Eighth Form: Moon-Dragon Ringtail – A massive circular slash that cuts through long-distance targets.
Ninth Form: Waning Moon Swaths – Intricate, overlapping slashes that drop from above.
Tenth Form: Piercing Phase Slash, Ivy Moon – A triple-layered saw-like attack.
Fourteenth Form: Catastrophe, Tenman Crescent Moon – A chaotic, omni-directional storm of giant moon blades.
Sixteenth Form: Moonbow, Half Moon – The ultimate long-range attack, raining down massive vertical slashes that crater the ground.

Note: Some forms (like the 4th, 11th, 12th, and 13th) were not explicitly named in the source material, but the progression suggests a total of sixteen.

The Final Reunion: Sixty Years Later

One of the most poignant moments in Michikatsu’s life occurred sixty years after he became a demon. He encountered Yoriichi one last time. To Kokushibo’s shock, Yoriichi was not only alive but was an old man—proving he had defied the Curse of the Mark.

Despite being over 80 years old, Yoriichi moved with a speed Kokushibo couldn't track. Yoriichi wept for his brother, pitying the monster he had become. In the ensuing duel, Yoriichi nearly decapitated Kokushibo in a single strike. However, Yoriichi died of old age mid-swing, standing upright. Enraged by the fact that he could never truly defeat his brother—even when his brother was a frail old man—Kokushibo desecrated Yoriichi’s body, only to find the handmade flute he had given him decades ago hidden in Yoriichi’s robes. Even after the betrayal, Yoriichi had cherished his brother's gift.

The Battle in the Infinity Castle

In the final arc of the series, Kokushibo faced the combined forces of the Demon Slayer Corps' strongest: Gyomei Himejima (Stone Hashira), Sanemi Shinazugawa (Wind Hashira), Muichiro Tokito (Mist Hashira), and Genya Shinazugawa.

This battle showcased the terrifying gap between Upper Rank One and the rest of the Upper Moons. Kokushibo fought with a cold, analytical precision. He recognized Muichiro as his own descendant, a child of the bloodline he had abandoned. Despite the Hashira's incredible teamwork and their awakening of the Transparent World and Red Nichirin Blades, Kokushibo pushed them to the brink of death.

He eventually underwent a further transformation, regrowing his head and becoming a monstrous, multi-limbed entity. However, at the moment of his "victory," he saw his reflection in the blade of a Hashira. He saw a hideous monster, far removed from the dignified samurai he once aspired to be. This realization, combined with the memory of the flute and Yoriichi’s smile, caused his resolve to crumble. His body began to disintegrate, unable to maintain its form once his spirit had finally admitted defeat.

What is the connection between Michikatsu and Muichiro Tokito?

Muichiro Tokito, the Mist Hashira, is a direct descendant of the children Michikatsu left behind when he became a demon. During their fight, Kokushibo noted the strength of the Tokito bloodline, expressing a twisted sense of pride in Muichiro’s talent. This connection adds a layer of generational tragedy to the fight; Muichiro represents the "human" path Michikatsu could have taken, choosing self-sacrifice and legacy over the hollow pursuit of personal immortality.

Conclusion

Michikatsu Tsugikuni is a cautionary tale of how the pursuit of greatness, when fueled by envy rather than purpose, leads to the destruction of the self. He had everything a man of the Sengoku era could want—family, status, and elite talent—but he threw it all away because he could not accept being second to his brother. As Kokushibo, he spent 480 years refining a sword style that, in the end, could not protect him from the weight of his own regrets.

Summary of Key Facts

Human Name: Michikatsu Tsugikuni.
Demon Name: Kokushibo (Upper Rank One).
Breathing Style: Moon Breathing (16 Forms).
Primary Motivation: Envy of his twin brother, Yoriichi.
Legacy: Ancestor of Muichiro and Yuichiro Tokito.
Weapon: A flesh-based katana that changes shape and size.

Frequently Asked Questions

Why did Michikatsu have six eyes as a demon?

His six eyes are a physical manifestation of his desire to see everything and possess the "Transparent World" ability at all times. It reflects his obsession with perception and his need to never be surprised by an opponent again, specifically Yoriichi.

Could Michikatsu have beaten Yoriichi if he had more time?

No. The series suggests that Yoriichi was a divine anomaly. Even as an 80-year-old man, Yoriichi was faster and stronger than Kokushibo at his peak. The gap between them was not a matter of training, but of fundamental nature.

What happened to the flute Michikatsu gave Yoriichi?

Yoriichi kept the flute until his death. After Kokushibo killed the deceased Yoriichi, the flute was sliced in half. Kokushibo kept the broken pieces of the flute with him for centuries, symbolizing that despite his hatred, he could never truly let go of his love for his brother.

Did Michikatsu regret becoming a demon?

In his final moments, yes. As his body crumbled, he questioned why he was ever born and admitted that he only wanted to be like Yoriichi. His last thoughts were of the brother he spent his life trying to surpass, acknowledging the futility of his 480-year existence as Kokushibo.