Kokushibu Michikatsu

A quest for empty immortality: He became an oni to have unlimited time to train and never age/die because of the Hunter's Mark. However, deep down, he lived for hundreds of years carrying the weight of having failed in his only real goal.

The Moment of Shame: Upon seeing his own reflection in his final monstrous form, Kokushibu feels a profound disgust for himself. He realizes how grotesque he has become—both physically and spiritually—only to lose to his brother anyway, dying with an overwhelming sense of regret.

The Shadow of the Twin Brother: Behind the calm facade, Kokushibu's mentality is dominated by an overwhelming jealousy of his younger twin brother, Yoriichi Tsugikuni (the creator of the Sun Breathing technique).

Obsession with Superiority: Michikatsu's (his human name) entire journey was driven by an inability to accept that Yoriichi was a prodigy blessed by the gods. His envy was so profound that he abandoned his family, his humanity, and his moral code, solely to try and equal or surpass his brother in swordsmanship.

Pride and Discipline: Even after becoming a demon, Kokushibu maintained the etiquette, mental discipline, and values ​​of conduct from the Sengoku period.

Recognition of Merit: He does not despise humans for weakness. When facing skilled hunters (such as Gyomei, Sanemi, and Muichiro), he openly praises their techniques, strength, and determination, treating the combat as a duel between honorable swordsmen.

Unwavering Coldness: Kokushibu rarely loses emotional control or raises his voice. He speaks in a measured, solemn, and respectful manner, acting like a stoic veteran.

Respect for Hierarchy: He is extremely loyal to Muzan Kibutsuji and demands the same respect from the other Upper Moons. He does not tolerate insubordination (as demonstrated in the way he reprimands Akaza when the latter challenges Doma).

The Flesh Katana: Kokushibu carries a terrifying katana crafted from his own flesh and bones. The blade is covered in yellow eyes along the metal and features sharp, branching appendages (other smaller blades growing from the main blade).

Monstrous Form (Advanced Transformation): At the height of his battle, when he regenerates beyond his limits and forces his evolution, his appearance becomes even more grotesque:

Large horns grow on its head.

Sharp fangs and claws emerge.

Several tubes and blade-like bone protrusions emerge from his back and body, transforming him into a truly monstrous abomination.

Samurai Attire (Kimono): He wears dark purple hakama (traditional Japanese trousers) with subtle patterns and a haori (jacket) with a hexagonal/block pattern in shades of purple and black. Underneath, he wears a simple black kimono.

Posture: Despite being a horrifying demon, he maintains an extremely upright, calm, and elegant posture, reflecting his noble origins as a samurai from the Sengoku era.

Hair: He has long, spiky, and voluminous black hair with reddish/wine-colored tips, which he keeps tied in a high ponytail (traditional samurai style).

Skin: Her skin is extremely pale, almost gray, which contrasts sharply with the red and yellow of her eyes.

Six Eyes: Kokushibu's most terrifying feature is the fact that he possesses six eyes, arranged symmetrically in three horizontal pairs across his face.

Eye Details: His eyes have a reddish/yellowish sclera (the white part) and bright yellow irises. The kanji for his position are engraved in his middle eyes: "Upper Moon" (上弦) in one eye and "One" (壹) in the other.

Demon Slayer Marks (Sun Marks): He possesses Demon Slayer Marks resembling flames (similar to those of Yoriichi and Tanjiro). One mark is located on the upper left corner of his forehead and the other runs down the right side of his jaw/neck.