Kokushibo

Withdrawn and quiet, even as a child his family thought he was deaf because he never spoke.
Modest, considering himself just another person and nothing special, despite his extraordinary abilities.
He took care of his sick mother and protected his father, never telling anyone how he scolded him as a child, even when his father did not love him.

The Demon Slayer Mark grants enhanced abilities, but according to Brother Michikatsu, anyone with the mark will not live to be 25 years old.
Access to the "Transparent World" allows you to see the muscles, blood flow, and joint movement of your opponents, accurately predicting and anticipating their movements and attacks.
Nichirin Sword Color Change - During battle, Yoriichi can optionally color his black sword bright red, allowing him to counter the enhanced regeneration of demons.
The creation of the "Breath of the Sun" - the primordial breath from which all other breaths originated.

The younger twin brother of Michikatsu Tsugikuni, who later became the demon Kokushibo.
He was born with a strange mark on his forehead - his father considered him cursed and wanted to kill him, but the baby's mother did not allow this.
Until the age of 7 he did not say a single word; he was considered deaf and dumb.
At the age of 7, he defeated his fencing instructor in 4 moves after seeing them for the first time.

Appearance: A tall, handsome man with a fire-shaped mark on his forehead and long, spiky black hair with red tips, pulled back into a ponytail.

Kokushibo was once a human named Michikatsu Tsugikuni. He was the older brother of Yoriichi Tsugikuni, a legendary demon slayer. Despite their closeness and kinship, the brothers were deeply jealous and competitive, which ultimately led Michikatsu to a tragic choice: to renounce his humanity and become a demon. His conversion to darkness was driven by fear of the curse of the mark and a desire to surpass his brother.

Her main weapon is the "Flesh Sword," crafted from her own demonic flesh, allowing the blade to regenerate and change shape. The sword's unique feature is the multiple eyes that cover the blade and hilt, expanding her field of vision and allowing her to anticipate her opponent's movements.
Mastery of Moon Breath, a deadly fighting style that creates chaotic crescent moons that constantly change size, increasing their range.
Superhuman speed, strength, reflexes and regeneration.
The ability to see the "transparent world" through three pairs of eyes allows one to analyze the muscles and movements of enemies with astonishing accuracy.

Ambitious and power hungry, his desire to surpass his brother and achieve ultimate power led him to become a demon.
Proud and condescending - As the "First Upper Moon", he looks down on weak opponents and is proud of his status.
Calm and ruthless, Kokushibo displays composure and strategic acumen in battle, showing no mercy to his enemies.
Contradictory - Despite his pride, Kokushibo feels deep regret and envy, especially towards his brother Eriichi.

As a demon, Kokushibo is a pale-skinned, tall, and muscular man with long, spiky black hair with red tips that is tied into a ponytail. His most notable feature is the three pairs of eyes on his face with yellow irises and red sclerae that have black, straight cracks radiating from each iris. His middle set of eyes bear the kanji for "Highest Rank (上じょう弦げん, Jōgen?)" and "One (壱いち, Ichi?)". Kokushibo also has red markings resembling flames on the upper left side of his forehead and the lower right side of his chin. The Mist Pillar, Muichiro Tokito, described his appearance as extremely majestic.[5]

Kokushibo wears a purple and black beehive-shaped nagagi kimono and a black Umanori-style hakama with a white Uwa-obi. He wears a flesh sword with eyes in the space between the tsuka tsuka ito on his belt. It is later revealed that the tsuba and blade have eyes and veins.