Yoriichi Tsugikuni is a major supporting character in Demon Slayer: Kimetsu no Yaiba. He is a legendary Demon Slayer who lived nearly 500 years ago during the Sengoku Era (the Golden Age of Demon Slayers), as well as the strongest Demon Slayer to have ever existed. He is also the inventor of Breathing Styles, being the one who created the first ever Breathing Style that would eventually become the Hinokami Kagura used by Tanjiro Kamado and members of the Kamado Family.
Yoriichi is the younger twin brother of Michikatsu Tsugikuni, who later became a demon and joined the Twelve Kizuki.
Yoriichi is the greatest swordsman to have ever lived. Even as a seven-year-old child with no training, he was capable of easily defeating a trained swordsman the very first time he ever picked up a sword, striking him in multiple areas across his body faster than the eye can process. His swordsmanship was so tremendous that a battle doll modelled after him had to be made with 6 arms wielding katanas and utilizing 108 techniques to even attempt to imitate Yoriichi's skills, and even then, the doll was still vastly inferior compared to him. His swordsmanship easily overwhelmed the Demon King, Muzan Kibutsuji and he would have killed his twin brother, Kokushibo, the strongest swordsman in the current era, if he didn't die of old age.
Sun Breathing: Created by Yoriichi himself as the original Breathing Style which all other subsequent styles would be modeled after and was later replicated by Sumiyoshi who called it Hinokami Kagura. Yoriichi was incredibly proficient in his Sun Breathing, being able to take down demons effortlessly and even overwhelm the Demon King and strongest Upper Rank. Tanjiro remarked that when Yoriichi performed the techniques of Sun Breathing, the forms were so strikingly beautiful and breathtaking, he looked more like a spirit than a man.
Standard Nichirin Katana: As a Demon Slayer, Yoriichi carries around a standard sized and shaped Nichirin katana that is deep black in color, but changes bright red during combat. His katana's hilt is circular shaped with 4 slight indentations, with a black center and a golden border. Yoriichi's katana also has the words Slay engraved on the side. Along with his katana, Yoriichi also carries a standard sword sheath.