Michikatsu and Yoriichi

Yoriichi Tsugikuni and Michikatsu Tsugikuni were twin brothers who lived during the Sengoku era. Yoriichi was born with the Demon Slayer Mark, which made him incredibly powerful. Michikatsu, though originally the heir to the family business, felt disadvantaged by his younger brother's talent.
After their mother's death, Michikatsu read her diary and learned that Yoriichi, as a child, had known of her illness and supported her in her final moments, despite considering himself weak. This discovery engendered a deep envy in Michikatsu, which over time grew into hatred. As a child, Yoriichi literally idolized his older brother. When Michikatsu gave him a homemade flute (or pipe), it delighted the boy. Later, Yoriichi even aspired to become the second most powerful samurai in the country—precisely out of respect for Michikatsu. Later, when Michikatsu, trying to reach his brother's level, abandoned his family and became a demon hunter, Yoriichi did not turn his back on him. He helped his brother master techniques—in particular, the "Moon Breathing." But there was a downside. Michikatsu envied his younger brother: Yoriichi had demonstrated phenomenal abilities since childhood (for example, at age seven, he easily defeated an experienced fencing instructor, even though he had never even held a sword). This envy fueled Michikatsu's internal conflicts.

All events take place during the Sengoku era. During the Sengoku period, the Demon Slayer Corps was at the height of its power. It was then that Yoriichi Tsugikuni, Michikatsu's younger twin brother and the most powerful slayer in history, lived. He created the Breath of the Sun technique, from which all other breathing styles—water, flame, wind, stone, thunder, and their derivatives—were derived, including the Moon Breath, which Michikatsu himself mastered.

Michikatsu lived almost five centuries ago, during the Sengoku period. He was the older twin brother of Yoriichi Tsugikuni. The brothers' childhoods were different. When they were born, their father noticed a strange birthmark on Yoriichi's forehead and wanted to kill him, believing him cursed. But their mother intervened, and it was ultimately decided that Yoriichi would enter the temple at age ten. Michikatsu secretly supported his brother, for example, by giving him a homemade flute.
Over time, Michikatsu began to envy his younger brother. Yoriichi had demonstrated remarkable abilities since childhood: he easily defeated experienced swordsmen and also possessed a special vision—he could discern the structure of living beings. Michikatsu feared that his father would unlock Yoriichi's potential and reverse their roles.
Michikatsu later became a samurai, married, and had children. But one day, a demon attacked his camp. Michikatsu was the only survivor, and Yoriichi came to his aid and apologized for not being able to save everyone. This episode finally pushed Michikatsu to his limits: he abandoned his family and became a demon hunter to train with his brother.
Michikatsu developed his skills, including training with a former Wind Breath user. He even awakened the Demon Slayer's Mark and created his own style, Moon Breathing, based on Yoriichi's Sun Breathing. But even this achievement didn't bring complete satisfaction: Michikatsu was never able to surpass his brother's genius. Michikatsu Tsugikuni is the real name of Kokushibo, one of the key characters in Demon Slayer Blade (Kimetsu no Yaiba). I've compiled a detailed description of his appearance for you—separately for his human and demonic forms, as the differences are significant. In human form
Height: about 189.5 cm.
Weight: approximately 93 kg.
Hair: Black, fairly long, with a spiky texture and slightly reddish tips.
Eyes: In human form, the iris is dark purple, the sclera is white.
Body type: strong, muscular.
Clothing: In the past, as a demon slayer, he wore traditional clothing.

Yoriichi is a legendary demon slayer who lived during the Sengoku era. He is considered the strongest demon slayer to have ever lived. Born with the Demon Slayer Mark, Yoriichi granted him superhuman abilities, access to the "Transparent World," and a state of selflessness. Yoriichi lived during the Sengoku era. He is considered the strongest demon slayer to have ever lived. He created the "Sun Breath," the original technique from which all other breathing styles in the world of "Blade" originated. Moreover, he was the first person born with the Demon Slayer Mark. He also had the "Transparent World" ability developed from birth: it allowed him to see through living beings—their internal organs, muscles, and bones. Thanks to this, Yoriichi could anticipate his opponent's movements and identify their weak points. Another of his characteristics was his "State of Detachment" (or "Selfless State")—it emitted no killing intent, aggression, or hatred, making him extremely difficult to detect or predict. Yoriichi was a tall, stately man with fair skin and dark burgundy eyes. He had long, wavy black hair with red tips, pulled back into a ponytail, with shorter bangs framing his face. On his forehead was a blood-red flame-shaped mark that spread across the left side of his forehead and temple. His preferred attire consisted of a pale orange kimono, black hakama, red haori, white tabi, and zori with red straps. Despite his immense strength, Yoriichi was a deeply humble man. He did not consider himself special and acknowledged his own limitations—for example, when he feared for his life during the battle with Muzan. At the same time, he was calm, reserved, and respectful. As a child, he was withdrawn: he didn't speak until he was seven, and those around him even thought he was deaf. He idolized his older brother, Michikatsu.