Yoriichi looked identical to Kokushibo when he was Michikatsu Tsugikuni, although there were notable differences. He was slightly more slender and had shorter bangs and wavier hair than his brother, colored black with red tips (burgundy in the anime, similar to Tanjiro Kamado). His Demon Slayer Mark was at the same place at the top of his forehead as Tanjiro, albeit lighter in color.
Yoriichi wore a red haori over an orange kimono with black hakama and a katana carried at the waist, conventional to most Demon Slayers. He sported the same hanafuda earrings passed down to Tanjiro Kamado from Tanjuro Kamado.
Yoriichi was a calm and respectful individual. He was extremely humble, seeing himself as just any other man and no one special. Yoriichi was also noted by his brother to always have had faith in the future and was confident that, despite the already overwhelming power the Demon Slayers of the First Breathing possessed, a generation would emerge that would further surpass their own. Normally, Yoriichi shows little to no emotion. However, Yoriichi does express emotions such as sadness, when he met his brother again during the final moment of his life, or when the baby Sumire want to hug him.
As a child, it was shown that Yoriichi adored and idolized his older brother, Michikatsu, even declaring that he would support his brother's dream of becoming the strongest Samurai in the land, and Yoriichi himself would become the second strongest. In his childhood, he was also extremely reserved and quiet, even being thought deaf by his family for seven years as he never spoke.
When he met his older brother again after sixty years, the latter having embraced his new life as a Demon, Yoriichi was deeply devastated and pitied his brother for what he had became. He attempted to kill Michikatsu himself in order to stop him as a traitor of the Demon Slayer Corps. However, despite his brother's betrayal, Yoriichi still treasured the handmade flute he gave to him, showing that he still loved and enjoyed his memories with him.
Abilities and Powers
Yoriichi is the strongest known character in the entire history of the series and the most powerful swordsman and Demon Slayer, being born with the Demon Slayer Mark and Transparent World. As a child untrained in swordsmanship, he was able to defeat a trained sword instructor and he was referred to as a miraculous prodigy. After training as a Demon Slayer, Yoriichi could defeat multiple Demons to where his older brother stated that he as a child couldn't even compare to him as an adult. He alone among the Demon Slayer Corps was able to corner Muzan Kibutsuji and had him on the verge of death until he managed to escape. Yoriichi was considered to be a real monster by Muzan, both the progenitor and strongest among all Demons.
Shortly before his death of at least 85 years in age, he battled his older twin brother, Kokushibo who turned into a demon. Despite both his extreme age and apparent blindness, he quickly gained the upper-hand in the battle in a single move, even slitting his throat before he finally died standing up, unscathed. Kokushibo himself admits that if Yoriichi launched a second swing before dying, he would have killed him while commenting Yoriichi's power was still in his prime.
- Demon Slayer Mark: A special marking that appears on strong Demon Slayers. Its specific abilities are unknown, but it drastically improves the abilities of the Demon Slayer, making them much stronger and faster with a quicker reaction than can be achieved normally, though at the cost of being cursed to die at the age of 25. Unlike all other marks users in his era, Yoriichi was able to defy fate and lived to about 85 years of age. It is presently unknown why Yoriichi is immune to the Mark's effects. Unlike all other mark bearers, he was born with this ability naturally.
- Transparent World (
透き 通る 世 界Sukitōru Sekai?) - Yoriichi had the ability to see the "transparent world", allowing him to see the muscles, blood flow, and joint movement of his opponents. He could accurately predict and anticipate the movements and attacks of his opponents with this ability. Unlike all others users, he was born with this ability naturally.
- Master Swordsman: Yoriichi was an extremely powerful and skilled swordsman. Even as child with no training, he was capable of easily defeating a trained swordsman the very first time he ever picked up a sword. His swordsmanship was so tremendous that a battle doll was made to look like him and have 6 arms wielding katanas utilizing 108 techniques only to imitate Yoriichi's skills.
- Immense Speed: Yoriichi possessed immense levels of speed, with which he was able to move fast enough to slit Kokushibo's throat without the latter being able to counter him, despite him being on guard and Yoriichi's blade was slightly near his throat.  Sometime before, during his battle against Muzan, he also managed to near-fatally injure Muzan with one move, despite the Demon's own inhuman speed and strength, and, when Muzan divided himself into 1,800 flesh pieces to escape, Yoriichi still managed to reduce the number down to a mere 500 in only seconds.
- Immense Endurance and Durability: As Yoriichi managed to survive all the way to his 80's- 90's years of age without his body succumbing to his Demon Slayer Mark or him having lost any of his original strength despite his advanced age and blindness, its likely that Yoriichi possess immense superhuman durability and endurance to still be fully capable of fighting and killing demon despite already being an aged old man without his power waning.
- Crimson Red Nichirin Blade: When fighting, Yoriichi had the ability to dye his black sword to crimson red, letting him hamper the enhanced regeneration of Demons, even Muzan's instant regeneration, leaving permanent damage burned to his cells that haven't healed for hundreds of years.
- Lack of Presence: During their first battle together, Muzan noticed that Yoriichi emanated absolutely no sense of drive, hatred, anger, fighting spirit, animosity or even bloodlust despite being in mid-battle against him, rendering it practically impossible for Muzan to detect him or gauge how powerful he was upon first seeing him despite his immensely enhanced and practiced Demon senses. This caused him to severely underestimate Yoriichi, a judgement that nearly cost him his life, and the Demon Progenitor then deemed Yoriichi to be an even greater monster and more of a genuine Demon than he ever was.
Sun Breathing (
- Clear Blue Sky (
碧 羅の 天Heki-ra no ten?) - The user spins his body vertically to create a 360-degree slash.
- Raging Sun (
烈 日 紅 鏡Retsujitsu kōkyō?) - The user unleashes two horizontal slashes to hit or intercept an enemy from the front or side.
- Fake Rainbow (
幻 日 虹Gen'nichi kō?) - The user utilizes this technique to evade incoming attacks with high-speed twists and rotations, thus creating afterimages. The afterimages work most effectively on enemies with good vision as they will probably fail attack the afterimage instead of the actual body of the user.
- Fire Wheel (
火 車Kasha?) - The swordsman leaps behind the opponent and spins in the air while releasing an attack in a circular motion.
- Burning Bones, Summer Sun (
灼 骨 炎 陽Shyakkotsu en'yō?) - A circular slash that defends from imminent frontal attacks.
- Sunflower Thrust (
陽 華 突Yōkatotsu?) - A single thrust attack with the point of the blade.
- Solar Heat Haze (
飛 輪 陽 炎Hirin Kagerō?) - The user charges towards the target, attacking it with a haze covered slash that seemingly fails to hit, but actually does hit.
- Setting Sun Transformation (
斜 陽 転 身Shayō Tenshin?) - The user flips into the air to deliver an upended sword slash.
- Beneficent Radiance (
輝 輝 恩 光Kiki Onkō?) - The user spirals into the air to deliver a powerful slash to those caught in it.
- Dragon Sun Halo Head Dance (の い Nichiun no Ryū Kaburimai?) - Resembling a dragon, the user moves in circles to avoid enemy attacks and slashes then multiple times at the target.
- Flame Dance (
炎 舞Enbu?) - A two-combo strike which starts with a vertical slash and then a horizontal one right after.
- Thirteenth Form - A technique that is utilized by continuously performing all the twelve styles in succession, which is noted to be the only effective method of killing Muzan Kibutsuji.
- The bag in which Yoriichi carried his brother's flute seemed to have been made of the cloth from Uta, his late wife's kimono, due to the similar patterns.
- The relationship between Yoriichi and Michikatsu parallels the mythology of the Shinto deities Amaterasu and Tsukuyomi, respectively. Amaterasu is the most sacred of Shinto deities, the sun goddess. Tsukuyomi, the moon god, is her former husband and brother. After Tsukuyomi killed Uke Mochi, another shinto deity, Amaterasu refused to ever look upon him again, moving to another part of the sky and beginning the cycle of day and night. Like the deities, the siblings also feud and have chosen different paths (that of the Demon Slayer and the Demon), developing their respective breathings of the sun and moon.
- (To Kokushibo) "My sympathies, my brother."
- (To Sumiyoshi) "I am a man with no worth."
- (To Kokushibo) "Brother your dream was to become the strongest samurai in the country, right? I want to be like you as well. I want to become the second strongest samurai in the country."
- (To Muzan) "What do you think lives are?"