Kokushibo

Kokushibo is one of the main antagonists in the Japanese manga "Demon Slayer", originally named Michikatsu Tsugikuni, he is a former swordsman of the Demon Slayer Corps, a user of Moon Breathing, and the strongest "Upper Rank One" of the "Twelve Kizuki", with the characteristic of having six eyes. He turned to Muzan Kibutsuji because he envied his younger brother Yoriichi Tsugikuni's powerful talent, becoming one of the strongest demons.

Background and Identity
Human Period: Born into a samurai family in the Warring States period, he was Michikatsu, and had a talented twin brother, Yoriichi Tsugikuni.
Reason for turning into a demon: In order to pursue power beyond Yoriichi's, Michikatsu chose to become a demon and changed his name to Kokushibo.
Relationship with the Demon Slayer Corps: He was once a former swordsman of the Demon Slayer Corps, and co-created the Breathing Techniques with Yoriichi, but later turned into a demon due to envy.

Abilities and Characteristics
Rank: "Upper Rank One" among the Twelve Kizuki, the most powerful demon besides Muzan.
Appearance: Has six eyes on his face, with the characters "一" and "Upper Rank" on the central eyes.
Weapon: Uses "Hokoku Shingo", made from his own bone marrow and blood, and the sword is covered with eyes.
Breathing Technique: Uses "Moon Breathing", derived from Sun Breathing.

Importance in "Demon Slayer"
He is the number one enemy of the Demon Slayer Corps, and engages in fierce battles with the protagonist Tanjiro Kamado and others in the Infinity Castle.
His tragic life and extreme pursuit of power demonstrate the distortion of humanity in extreme environments.