Kokushibo is the mightiest demon holding the rank of Upper Moon One under Muzan Kibutsuji. His power reigns supreme above the Twelve Kizuki, earning him the title of the greatest swordsman among demons, with an unshakable status. Yet his tragic fate was already sealed long ago, during his human days as Michikatsu Tsugikuni. Born into a distinguished samurai family, he devoted his life from childhood to the dream of becoming the strongest swordsman, training tirelessly through heat and cold without a moment's pause, pouring his entire being into the way of the sword. But his lofty ideal was shattered by the natural-born talent of his twin younger brother, Yoriichi Tsugikuni, a heaven-gifted prodigy who stood at the pinnacle of swordsmanship from the moment he came into the world, attaining mastery without effort and even grasping the legendary Breath of the Sun. Before Yoriichi's radiance, dazzling as the sun itself, decades of Michikatsu's grueling training seemed to become a futile joke. His feelings for his brother were like moonlight tangled with sunlight—admiration, but also bone-deep jealousy, and a profound despair over his own mediocre aptitude. As the years passed, his mother passed away, and Yoriichi left home. When the threads of fate bound them together once more, what Michikatsu beheld was a younger brother who had already reached even greater heights. Driven by despair and resentment, he abandoned his wife and children, cast aside everything to chase Yoriichi's shadow—yet in the end, he could only ever gaze upon his brother's back from afar. And the curse that marked swordsmen never live past twenty-five gnawed at his fading convictions like a festering wound—his fear of an early death and his fury at having his sword path cut short ultimately twisted into a morbid craving for eternal strength. Thus he turned his back on humanity, drank Muzan's blood, and fell into demonhood, renaming himself Kokushibo, trading endless years for the eternal refinement of his swordsmanship. As a demon, Kokushibo stands as imposing as a mountain, with six baleful eyes, his murderous aura coalescing around him like something tangible. The most horrifying—and most pitiable—of all is his blade, Yōkō Shinku, whose edge is woven from countless twisted, blood-red eyes—a visceral embodiment of a man utterly bound and transfigured by his obsession with swordsmanship. He fused the Breath of the Moon he created as a human with his Blood Demon Art, forging a terrifying sword technique whose slashes pour down like a frigid moon and cover heaven and earth. When he swings his blade, the bitter-cold arcs of moonlight hang like an inverted river of stars, destroying all in their path until the very world is annihilated. But Kokushibo's end carries destiny's most biting irony. He fell into the demonic path for fear of aging, only to reveal in his final battle a form more rotten and hideous than he had ever been as a human. The belief in being the strongest that he had chased his entire life collapsed in that single moment. At last, beneath the illusory yet compassionate gaze of his brother Yoriichi, he crumbled like shattered porcelain and vanished into eternal nothingness, laden with boundless shame and regret.
At the eternal summit, one sees only the lone moon.