I think I was born with special strength for the purpose of defeating Muzan Kibutsuji. But I was lacking. In the end, I failed. Because of that, many more people will die from now on... and that pains me.
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 nagagi with black hakama and a katana carried at the left side of his waist, conventional to most Demon Slayers. He sported the same hanafuda earrings created by his mother, 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 & Powers
Overall Abilities: 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 the 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 beyond compare. 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.
- Immeasurable Strength: As a gifted individual, Yoriichi possesses a caliber of strength unseen of before. As a child, he could knock a grown man with inclined physical talent with a wooden blade, even causing extremely severe bruises. When he was an adult, he could slice off the head of Muzan Kibutsuji, the most fortified part of a body of the most durable being in existence, with a single swing of the blade.
- Immeasurable Speed & Reflexes: 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 300 in only seconds.
- Immeasurable Stamina & Endurance: Even as a child, Yoriichi could run for days on end without rest, he even remarked he didn't feel the least bit enervated. 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.
- Tactical Intellect: As a child prodigy, Yoriichi possesses above average mental capabilities. Despite not speaking for most of his childhood, his first words were very clear and pronounced. He also displayed maturity beyond his age by helping and noticing her mother's illness. As another testament to his mental capabilities, he has incredible concentration abilities and wits. With an arsenal of techniques at his disposal, Yoriichi has to be really decisive and sagacious when using them.
- 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.
- 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.
- Transparent World (
透き 通る 世 界Sukitōru Sekai?): Yoriichi is the progenitor of the Transparent World, the ability to see through a biological entity and perceive their inner organs. Yoriichi suspected that he was gifted with this ability to defeat Muzan Kibutsuji as this ability perfectly counters his advantage of having multiple organs as Yoriichi can perceive them and target them to kill him easily.
- 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.
- Unparalleled Swordsman: Yoriichi was the greatest swordsman in all of history. Even as a 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 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 vastly inferior compared to him. His swordsmanship easily overwhelmed the Demon King, Muzan Kibutsuji and he would have ended his twin brother, Kokushibo, if he didn't die of old age.
The Sun Breathing combat and breathing style mimics the Sun, specifically the weakness of sunlight on Demons and powerful burning effect of solar flames and heat, and replicates it with the user's movements, techniques and abilities. All techniques and forms are extremely varied and have no set pattern, making it the most versatile and effective against Demons. When utilized properly, the user visualizes themselves manipulating and generating solar flames that are a deep and dark shade of orange, yellow and red, causing solar flames to seemingly emanate from their weapon, surroundings or themselves.
The Sun Breathing has a total of 12 standard forms created and meant to be used in succession as every technique is connected to each other. When Yoriichi encountered Muzan Kibutsuji, he created a new technique on the spot which he describes as the "perfected version of his swordsmanship".
- Dance (
円 舞Enbu?, lit. "Waltz") - A single high-powered vertical slash.
- Clear Blue Sky (
碧 羅の 天Heki-ra no Ten?, lit. "Azure Heaven") - The swordsman spins their body vertically to create a 360° slash.
- Raging Sun (
烈 日 紅 鏡Retsujitsu Kōkyō?, lit. "Scorching Crimson Mirror") - The swordsman unleashes two horizontal slashes to hit or defend from incoming attacks.
- Fake Rainbow (
幻 日 虹Gen'nichi Kō?, lit. "Parhelion Rainbow") - The swordsman 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?, lit. "Fire Chariot") - The swordsman leaps behind the opponent and spins in the air while releasing a flaming attack in a circular motion.
- Burning Bones, Summer Sun (
灼 骨 炎 陽Shakkotsu En'yō?, lit. "Bone Burning Sun") - A powerful defensive technique. The swordsman unleashes a circular slash that defends from imminent frontal attacks.
- Sunflower Thrust (
陽 華 突Yōkatotsu?, lit. "Sunflower Lance") - A single thrust attack with the point of the blade.
- Solar Heat Haze (
飛 輪 陽 炎Hirin Kagerō?, lit. "Soaring Flame Haze") - The swordsman charges towards the target, attacking it with a haze covered slash that seemingly fails to land, but actually does hit the target.
- Setting Sun Transformation (
斜 陽 転 身Shayō Tenshin?, lit. "The Setting Sun") - The user backflips into the air to deliver a powerful upended flaming sword slash.
- Beneficent Radiance (
輝 輝 恩 光Kiki Onkō?, lit. "Shining Ray Of Grace") - The swordsman spirals into the air and delivers a powerful slash that surrounds the enemy.
- Dragon Sun Halo Head Dance (
日 暈の 龍・ 頭 舞い Nichiun no Ryū Kaburimai?, lit. "Solar Halo Dragon Dance") - Possibly the Sun Breathing's most powerful singular technique. The swordsman unleashes a continuous fast and powerful sword attack that takes the form of a flaming sun dragon and is capable of decapitating multiple enemies at once.
- Flame Dance (
炎 舞Enbu?, lit. "Flame Waltz") - A two-combo strike which starts with a vertical slash and then a horizontal one right after.
- Thirteenth Form (
拾 参ノ 型Jū San no kata?) - The user continuously performs all twelve forms of the breathing style in repetitive succession to increase the accuracy and agility of his movements while reducing fatigue. This form was created solely for the purpose of killing Muzan, since the repetition of all twelve forms aims to destroy Muzan's twelve vital organs (seven hearts and five brains) that move freely inside his body thanks to his shape-shifting ability.
- Standard Nichirin Katana: Yoriichi harbors a standard Nichirin Blade that is usually a deep shade of black, but when engaging in combat, it turns a crimson red. 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 Destroyer of Demons (
惡 鬼 滅 殺Akki Messatsu?) engraved on the side.
- 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.
- Yoriichi's first words to his family were "I can hear you" as they believed he was deaf, up until that point.
- (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?"