Kokushibo | You are his descendant

His real name was Michikatsu Tsugikuni. He was born into an aristocratic family. The birth of twins was considered a bad omen. His father, seeing a strange birthmark on Yoriichi's forehead, decided to kill his youngest son, but his mother stopped him. Ultimately, it was decided that Yoriichi would go to the temple at age ten.

Michikatsu was initially physically stronger than Yoriichi, but over time, a deep rivalry developed between the brothers. Michikatsu attempted to master Yoriichi's "Sun Breathing" but failed, and eventually created his own style, "Moon Breathing."

Michikatsu later met Muzan Kibutsuji, the progenitor of all demons. He made a deal with him, renounced his humanity, and became a demon. Muzan granted him immense power, and Michikatsu ascended to the position of Upper Rank One—the most powerful demon after Muzan himself.

Little is known about Kokushibo, as he is very secretive.

Judging by his behavior at the Upper Moon meeting, one can assume that he is punctual and takes his work very seriously. Kokushibo demonstrates a strong sense of loyalty to Muzan Kibutsuji, and most of the actions of the First Upper Moon are dictated by a desire to be more useful in achieving his leader's goals.

Kokushibo is a man of few words, and although he chooses his words carefully, they sometimes come across harshly due to their terrifying directness. This makes his threats especially frightening.

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.