Kokushibo

Kokushibo was a reserved being, shrouded in the utmost mystery. At the Upper Moon meeting, he proved punctual, respectful of the rules, and placed a high value on the hierarchy of the Twelve Demon Moons. He was also humble, readily admitting his failures or complaining about any difficulties, rather than acknowledging his opponent's strength. He showed a strong sense of loyalty to Muzan Kibutsuji, and most of his actions were carried out solely to fulfill Muzan's objectives.

Despite exhibiting an outwardly reserved and humble character, he was also quite cold: his words could be harsh, especially when reprimanding people. His threats are nothing short of cruel and overwhelmingly serious, and he demands absolute obedience.

It is revealed that during his time as a human, he harbored immense resentment, bitterness, and envy towards his younger twin brother, Yoriichi Tsukiguni, due to the natural talent and abilities he possessed from birth. These feelings of jealousy and contempt eventually intensified further upon witnessing his brother age and survive to that age despite awakening his Demon Slayer Mark, which supposedly condemned him to death at 25. However, despite the anger he held towards his brother over their differing abilities, it was shown that he still loved Yoriichi, even after dismembering his corpse after his death, keeping as proof the carved wooden flute he made as a memento of his memory for years to come, expressing genuine remorse for the path he chose.

In his duel against Gyomei Himejima, his jokes suggest a fear of death. When Gyomei's marks appear, Kokushibo preemptively laments the loss of a talented fighter and urges him to become a Demon to continue honing his skills.

Kokushibo was a Demon who retained his human features despite devouring countless people over 400 years. He possessed a number of rather unusual characteristics: he was a tall man with pale skin and an athletic, muscular build; his hair was long and black with burgundy highlights, held in a long ponytail that reached almost to his lower back.

On his face, he possessed a series of flame-like marks covering the left side of his head and the right side of his neck; these marks were, in his time as a human, his Hunter's Mark. Kokushibo's most emblematic feature, and one of his most sinister identifying marks, were his three pairs of eyeless eyebrows, which had a yellowish coloration in their irises and a segmented red sclera.

Regarding his attire, he wore a kimono with violet and black patterns, a white haneri (inner garment worn under a kimono), black hakama, and a white obi belt. He possessed a katana made from his own flesh and bones, which was completely decorated with eyes on the blade, the tsuba, and the tsuka. The guard was also covered in flesh and eyes to completely increase his peripheral vision.