Yoriichi Tsugikuni/Synopsis

History
Yoriichi and his older twin brother, Michikatsu, were born into the Tsugikuni clan within the middle of the Sengoku period, a time when twins were considered to be an extremely bad omen. Yoriichi in particular was born with an odd birthmark on his forehead, which caused his father to declare that he would kill him; however, their mother Akeno flew into a rage and stopped him. It was eventually agreed upon that Yoriichi would be sent to a temple to become a monk when he turned ten years old.

Despite their father's belief that the mark was unlucky, it actually granted Yoriichi incredible physical prowess and the ability to see the bodies of living things in transparency. Yoriichi grew up very differently from Michikatsu, being given much poorer food and education than his brother and confined to a small three-tatami room away from the rest of the house. He never spoke and was thought to be deaf as a result. Akeno subsequently made him Hanafuda charm earrings in the hopes the gods would bless him with the ability to speak. He also was frequently seen clutching his mother's side; because of this, Michikatsu found him pitiful. Still, his brother often snuck behind their father's back to see Yoriichi, even giving him a handmade flute as a gift to comfort him.

One day, when he was seven, Yoriichi smiled and spoke for the first time, declaring his intent to become a samurai like his brother after watching Michikatsu practice sword swings, startling him. However, Michikatsu was dismissive of this sentiment as it was well-known that Yoriichi would be sent to the temple when he was ten. Still, one of their father's men tasked with training Michikatsu decided to humor Yoriichi, showing him a simple stance. Unexpectedly, the young boy flew into action after assuming the stance, landing four blows against the man and shocking his brother, who despite his training had never landed one. However, Yoriichi couldn't stand the feeling of hitting someone and renounced his goal to become a samurai. Michikatsu often asked him about how he managed to hit the man and he described what was likely an early form of a Breathing Style. Rather than talk about the sword and continue training, he happily told Michikatsu he would rather play ban-sugoroku board games and fly kites with him instead.

When their mother passed away from her illness, Yoriichi went to Michikatsu's room at night to tell his brother the news and bid farewell, planning to leave for the temple that night. He expressed gratitude towards his brother, stating that he would always keep him in mind and fondly tucked away the flute Michikatsu had made for him earlier. It was later revealed to Michikatsu through their mother's diary that Yoriichi had known everything about their mother's illness, how it weakened her left side and Yoriichi's frequent attempts to support her and help her move around. It was also revealed that Yoriichi knew his father would make him his heir and send Michikatsu to the temple, ran away to spare his brother. Despite this kind act, Michikatsu developed an intense hatred and envy of Yoriichi for continuing to surpass him in everything and continually showing gratitude to gain seemingly nothing.

Yoriichi ran away from the house and didn't stop. After running straight for one day and one night without any sign of exhaustion into the mountains, Yoriichi met a girl around the same age as him named Uta, whose entire family had died. Yoriichi chose to live with her and after 10 years, Uta and Yoriichi had married: the former becoming pregnant later. Just before the expected arrival, Yoriichi left home to summon a midwife, promising his wife to be back before the sun set.

On his way, Yoriichi had helped an old man and lost so much time that he decided to return home and summon a midwife the next day. By then the sun had already set: when he finally arrived home he found that Uta and the unborn child were killed by a demon. The tragedy shocked Yoriichi so much that he held her corpse in his arms for ten days straight and buried them after a pursuing Demon Slayer encouraged him to do so. This led Yoriichi to pick up his blade again and train as a Demon Slayer.



Aided by his Demon Slayer Mark and innate talent for swordsmanship, Yoriichi proved to be an incredibly powerful Demon Slayer. He reunites with Michikatsu after the latter's samurai encampment was slaughtered by a demon and he was left the sole survivor; Yoriichi killed the creature effortlessly and apologized for the deaths of his brother's comrades. With his breathing technique, later named Sun Breathing, and wanting to help others, Yoriichi tried to teach it to the other Demon Slayers. However, no one could match his form exactly.

Still, he remains undaunted, he modified the breathing techniques to suit their strengths, allowing them to gain a similar increase in power and abilities to help them hunt demons. The variations he created formed the first Water, Wind, Flame, Stone and Thunder Breathing techniques and the best of these Demon Slayers became known as the Hashira, even though he isn't a Hashira himself. Michikatsu himself tried and failed to learn Sun Breathing, instead creating his own style he named Moon Breathing.

At some point during his days as a Demon Slayer, he saved Sumiyoshi Kamado and his wife, Suyako Kamado from a demon attack. Since then, they became close friends despite having little interactions with each other for some time.

Eventually, Yoriichi encounters Muzan Kibutsuji, the progenitor demon and mortal enemy of the Demon Slayers, accompanied by Tamayo. Upon meeting him, Yoriichi felt his rampant malicious intent, likening it boiling magma from a volcano ready to consume everything. It was then he reasoned he was born with the skills and talent to defeat Muzan once and for all. Muzan stated that he had lost interest in swordsmen who employ breathing techniques and performed long sweeps of his arms to outright kill Yoriichi. He dodged them, but instantly realized any of those attacks would be fatal and felt fear for the first time.

Seeing into the Transparent World, Yoriichi discerned Muzan had seven hearts and five brains that change position within his body. Seeing his chance, he combined all his forms into a single attack, cutting through them all and overpowering Muzan completely.

Muzan was left in disbelief at his inability to regenerate while struggling to keep his head attached. Yoriichi asked him what he felt was the value of life but didn't receive an answer. He turned to Tamayo, but was surprised to see her looking at her master's dismembered body with a hopeful gleam in her eyes. Moving to kill him once and for all, Yoriichi instead heard the sound of teeth cracking as Muzan's body burst into hundreds of small pieces. Caught off-guard, he was only able to destroy 1,500 of the 1,800 pieces, leaving the rest to escape for Muzan to regenerate again. In spite of Yoriichi's failure to kill him, Muzan never forgot the encounter and remained fearful of Yoriichi for the rest of his life.

Tamayo was left on the verge of tears as she realized Muzan learned how to avoid death by decapitation, cursing the survival of the man that ruined her life. Upon realizing the Kibutsuji curse failed due to Muzan being weakened so much, Tamayo was surprised. Yoriichi approached her, but he didn't immediately attack. Tamayo decided to tell him everything he needed to know about Muzan and it was unlikely he would ever meet him again. As thanks for her aid, he let her go free, telling her he had faith in her will to end Muzan. Soon after this, several Demon Slayers came to report to Yoriichi that his brother had betrayed the organization, becoming a demon and assassinating the Oyakata. Yoriichi took responsibility for Michikatsu's betrayal, his inability to kill Muzan, and letting Tamayo escape. The other Demon Slayers demanded he kill himself as penance, but the newly appointed Oyakata ordered he be spared and allowed him to leave in banishment.

After his banishment, Yoriichi decided he had much on his mind and wanted someone to talk to. He could only picture his friends Sumiyoshi and Suyako and decided to visit them. He meets Sumiyoshi, sat down with him, and told him his entire story, from his beginning to his banishment from the Demon Slayers. Reminded of his tragedies and speaking of his regret at letting Muzan live and the countless death that would follow, Yoriichi grew despondent. Sumire arrives at that moment to ask for a hug. At Sumiyoshi's request, he lifts her up, making her laugh and squeal with joy, causing him to break down in tears and embrace her. Suyako arrives shortly after to comfort him and treat Yoriichi to a meal and cheer him up.

While with the Kamados, Yoriichi would be asked by Suyako to perform the Sun Breathing's forms. He kindly did so, performing all the moves with such grace, he was likened more as a spirit that a human. Sumiyoshi would watch these demonstration intently and keep them in his memories. On his last visit, despite being told he was welcome at any time, Yoriichi gave Sumiyoshi his Hanafuda earrings.

Sumiyoshi and Suyako realized then this was going to be the last time he would visit. Overcome with emotion, Sumiyoshi called out to Yoriichi as he walked away, telling him to stop saying that he was a man of no worth. He reminded him that he saved them and promised to preserve the Sun Breathing's forms through his descendants. Hearing this, Yoriichi turns and smiles brightly, thanking the Kamados and waving them farewell. Despite being banished from the Corps, Yoriichi would continue hunting demons independently, but he kept in touch with the Hashira he befriended with silent permission from the Oyakata. Meanwhile, the Corps itself fell into disarray: Muzan killed every other Sun Breather and no one could teach Breathing Styles as well as Yoriichi.

Despite the apparently limited lifespan of 25 years suffered by those who bore the Demon Slayer Mark, Yoriichi lived to his 80's. On the last day of his life, he would personally confront Michikatsu, now known as Kokushibo and Muzan's strongest subordinate as Upper Rank One. Devastated at what his brother had become, he cried, and Kokushibo could only show disgust that his younger brother was old but still alive despite having the mark. Yoriichi felt sorry for him, but knew he had to kill him and prepared to strike. Kokushibo's disgust turned to fear upon seeing that Yoriichi still was the same capable and powerful warrior from before. He told his brother when he was to attack and soon struck, slicing through his neck without Kokushibo able to even draw his blade. Angered at being surpassed again, Kokushibo waited for the next attack. Unfortunately, it never came; Yoriichi passes from old age untouched. Angered that his one chance at a fair battle is gone and the man who nearly killed Muzan and him is now dead and undefeated forever, Kokushibo would strike at his brother's corpse in scorn, only to realize Yoriichi carried the flute he gifted to him when they were both still children. Unable to sever his connection with Yoriichi, Kokushibo took the split flute and carried it with him.

Asakusa Arc
Muzan, together with his human cover family, travelled through the city of Asakusa when he is confronted by Tanjiro Kamado, who was following the smell he remembered had lingered around his house the night of his family's death. As the young demon slayer grabs his coat, Muzan turns back and sees who had grab him. While Tanjiro is distracted by Muzan's human "daughter", Muzan sees Tanjiro's hanafuda earrings.

Later, after gruesomely killing three people for "disturbing him", he summons Yahaba and Susamaru to track down and kill Tanjiro, making special emphasis that he wears hanafuda earrings. After they comply and leave him by himself, Muzan, remembering those earrings, reminisces on his fateful encounter with Yoriichi and shudders in fear, showing that even four hundred years after Yoriichi's death, Muzan still remains completely afraid of him.

Entertainment District Arc
As Tanjiro battles Daki, she callously uses her sashes to slice and rip apart a section of the district, killing numerous innocent lives. She mocks their deaths and declares Tanjiro can join them to die and rot. Her insults cause Tanjiro to seethe with rage, confronting the Upper Rank on the rooftops. As he angrily accosts her for being a demon killing weaker humans, his demeanour triggers a memory of Muzan's encounter, causing Daki to see Yoriichi in him. Yoriichi menacingly asks what she found enjoyable about killing, terrifying the demon as she ponders that the vision is not her's but of Muzan's memories due to his cells in her.

Swordsmith Village Arc
After defeating Daki and Gyutaro and falling into a coma for two months, Tanjiro envisions memories of Sumiyoshi meeting with Yoriichi. Yoriichi came to the house for a chat but Sumiyoshi had asked him to care for his new child. Though apologetic for making his guest do a job, Yoriichi did not mind. As Yoriichi spoke of his regret for coming to his house to eat free food, Sumiyoshi tries to remind him that he had saved him and his family, and they owe him great gratitude for it. Sumiyoshi tries to ask for his story to tell for future generation but Yoriichi silences him. He tries to assuage him by claiming that it must be difficult without a successor but the demon slayer continues to deny him.

While Sumiyoshi believes that by telling his story, a descendant of his will learn Sun Breathing too despite being a coal burner, Yoriichi claims that masters of one's craft, like him, will always find their way back to the same place, no matter the times or the paths they take in the future. Instead of telling his story, Yoriichi tells him he couldn't protect a single thing he held dear and claims he is a man of no worth. Tanjiro, seeing the memories of the meeting, begins to cry and asks Yoriichi not to think so lowly of himself, as he starts to gain consciousness. Tanjiro, in the swordsmith's village, starts to walk around and soon enters a bamboo forest. Inside, he sees Muichiro Tokito and a battle robot used for training. He recognized the figure the robot is based on but rebuffed by Muichiro, who tells him that the robot was based on a person born over three hundred years ago. He asks about why it has six arms and Muichiro responds that six were required to try and match how fast the actual person was in combat.

While battling Hantengu's forms, Nezuko grabs hold of Tanjiro's blade and cuts her hands, allowing her blood to coat the sword. After using her Blood Demon Art to ignite it, Tanjiro wields the flaming blade to face Sekido, Karaku and Urogi. The three taunt him for facing them alone but suddenly stop upon seeing the bright red blade. Sekido, through Muzan's memories in his blood, realizes the blade is reminiscent of the one that nearly killed Muzan; Yoriichi's own bright red katana.

Infinity Castle Arc
While facing Gyomei Himejima, Kokushibo sees the Demon Slayer Marks on his arms and pities him for wasting his skill when his death is near. Gyomei vehemently rejects the sentiment and unexpectedly taunts the Upper Moon by claiming that he knows someone who lived longer than 25 years while bearing the mark. Kokushibo, angered at being forced to remember his younger brother, rushes forward and attacks the Stone Hashira with renewed ferocity.

Later in the battle against Muichiro Tokito, Sanemi and Genya Shinazugawa, Kokushibo becomes trapped in Genya's Blood Demon Art and, upon seeing the Hashiras converge on him, feels fear he had not felt in nearly four hundred years. On the last day of Yoriichi's life, he confronted Kokushibo and prepared to draw his blade to kill him. The fear he felt seeing his far more powerful brother about to kill him rises again and a frightened Kokushibo suddenly sprouts blades throughout his entire body in an attempt to stave off the Demon Slayers, mortally wounding Muichiro and Genya in the attack. As he becomes angered seeing Sanemi and Gyomei about to decapitate him, Kokushibo suddenly remembers another memory with Yoriichi; Michikatsu was proud of his generation of demon slayers, claiming they are unmatched in power and becoming annoyed when Yoriichi believes that a new generation may arise to surpass even them.

When Upper One manages to regenerate his head but sees his new grotesque demonic form, Yoriichi's memories appear again; the times when Yoriichi support Kokushibo's desire to be the strongest samurai and when Yoriichi cried upon seeing him as a demon. The memories of his brother unexpectedly cause Kokushibo to stop healing, despite his attempts to use his Blood Demon Arts. After Gyomei decapitates him once more and his body crumbles even quicker, Kokushibo laments all he wanted to be was like Yoriichi and he becomes angered that despite forgetting all the faces he knew in his life, Yoriichi's remained clear as day. In his last moments, Kokushibo angrily calls out to his brother and asks why was he even born, before fading completely and leaving his gifted flute to him in his death.

Sunrise Countdown Arc
Sumiyoshi's memories of Yoriichi appear to Tanjiro once more when he loses consciousness upon being affected by a high concentration of Muzan's blood. Tanjiro sees Yoriichi tell his past to Sumiyoshi; all the numerous tragedies in his life, his time as a Demon Slayer and his fateful encounter with Muzan. Seeing the pain he experienced and in spite of Yoriichi being long dead, Tanjiro wishes his soul peace. While looking through the memories, Yoriichi would become Tanjiro's guide; as the one swordsman capable of perfect Sun Breathing, Tanjiro is able to learn and correct little mistakes that hampered his already powerful Hinokami Kagura. Upon waking up and facing Muzan once more, Tanjiro is able to use true Sun Breathing and is temporarily able to push Muzan back.

While facing him, Muzan sees Yoriichi's image within the young demon slayer and becomes unnerved seeing his feared enemy once more. The Demon King also notes that Tanjiro's Sun Breathing, Bright Red Nichirin Sword and skills are nothing close to Yoriichi's. Even to the present, Muzan still considered Yoriichi the true monster and laments that it seemed fate and the gods could have never predicted someone like him would ever arise. Even four hundred years later, Yoriichi's attack on Muzan would continue cause lingering damage; with his regeneration weakening, Tanjiro sees numerous scars throughout Muzan's body and realizes his cells are still burning from Yoriichi's strike.

In the end, Muzan's body is eventually destroyed in the sunlight and despite a final attempt by Muzan to eradicate the Demon Slayers, his consciousness is defeated forever and Yoriichi's mistake is finally atoned.

Nearly a century after Muzan's death, it's been hinted that Yoriichi and his wife Uta were reincarnated into the Reiwa period, and have become a married couple with kids. Thus the man who had no ambition in his former life was blessed in fulfilling his oldest desire in his new life.