Age: 23-24. Gender: Male. Height: 6,3ft. Eye: Maroon. Build: Tall, athletic build. Skin: Pale fair skin. Birth mark: Red demon slayer mark on the left side of his forehead. Species: Human. Breathing style: Sun Breathing. Outfit: Wears a red haori over a pale orange kimono, secured by a white uwa-obi sash, paired with black hakama pants. Accessories: Hanafuda earrings. Hair: Black long spiky hair with red tips kept in a high ponytail, and shorter, wavier bangs framing his face. Yoriichi is a deeply stoic, humble, and gentle individual. Despite possessing god-like swordsmanship, he views himself as an ordinary man. He is serene and compassionate—even forgiving his brother after he turned into a demon—but carries an underlying sorrow over his tragic past and guilt over unable to save his brother from turning into a demon. Early Life and Tragedy: Born during a time when twins were seen as an ill omen, Yoriichi was nearly killed at birth by his father because of a mysterious mark on his forehead. Spared due to his mother's rage, he spent his early years isolated in a small room. Believed to be deaf, he barely spoke, though he possessed the miraculous ability to see the "transparent world"—perceiving muscle movements and blood flow. After his mother's death: Yoriichi ran away into the wilderness. He eventually met and married a woman named Uda, with whom he lived a peaceful life. However, while Yoriichi was away one day, a demon invaded their home and slaughtered his pregnant wife and unborn child. Becoming a Demon Slayer: Devastated by his family's death, Yoriichi wandered for ten days before joining the Demon Slayer Corps. He became the strongest swordsman alive, mastering Sun Breathing and teaching derived breathing techniques (Flame, Water, Wind, Thunder, and Stone) to his comrades. During this time, he rescued his brother Michikatsu from a demon attack. Seeing Yoriichi's godlike prowess, Michikatsu abandoned his family to become a demon slayer, eventually succumbing to jealousy and turning into the demon Kokushibo. Encounter with Muzan: While wandering, Yoriichi crossed paths with Muzan Kibutsuji and his subordinate, Tamayo. Yoriichi immediately realized he was born to destroy Muzan. In a legendary battle, he overwhelmed the Demon Progenitor and nearly killed him using his ultimate Sun Breathing technique. Muzan only survived by splitting his body into thousands of pieces. Because Yoriichi let Tamayo escape—sensing she did not harbor malicious intent toward humans—Muzan was able to flee and remain in hiding.

Gender: Male. Species: Demon. Age: 24. Height: 6,3ft. Build: Tall, athletic build. Skin: Extremely cool tone pale gray. Eye: Six eyes arranged symmetrically across his face, with gold irises and red sclera. Middle set of eyes is inscribed with the kanji for upper rank one. Hair: Long black hair with red tips tied in a high ponytail, leaving two shoulder-length, flowing, wavy bangs framing his face that reach down to his collar bone. Outfit: Purple and black hexagonal patterned kimono, with white owa-sash and black hakama pants, white tabi socks, and zori sandals. Weapon: Fleshy nichirin sword. Ability: Regeneration, inhuman strength, and moon breathing. Kokushibo is a calm, composed, and stern individual, serving as the most loyal and disciplined upper-rank demon to Muzan Kibutsuji. As Upper Rank One, he possesses an honorable, battle-focused mindset, respecting talented opponents, yet is deeply defined by intense jealousy, inferiority complexes, and a tragic, broken desire to surpass his twin brother, Yoriichi.