Yoriichi Tsugikuni/Synopsis

History
Yoriichi and his twin brother, Michikatsu, were born into the Tsugikuni clan in 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 prevented him; it was eventually agreed upon that Yoriichi would be sent to the 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 even 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 Yoriichi described what was likely an early form of a Breathing Style, but rather than talking about the sword and continue training in the sword, he happily told Michikatsu he would rather play sugoroku 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 the moment she died, Yoriichi ran away in order to let him become the true succesor and avoid being sent to the temple. Despite this kind act, Michikatsu developed an intense hatred and envy of Yoriichi, considering him a genius, for continuing to surpass him in everything and for 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 became married and Uta became pregnant with his child. Just before the expected arrival of their child, Yoriichi left home to summon a midwife, promising his wife to come back before the sun set. On his way Yoriichi had helped an old man, losing so much time that he decided to return home and summon a midwife the next day. By then the sun then had already set and when he finally arrived home he found Uta and their unborn son laying on the ground, both killed by a Demon. The tragedy shocked Yoriichi so much that he held their corpses in his arms for ten days straight and only buried them after a pursuing Demon Slayer encouraged him to do so. This was the cause for 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. Aided by his breathing technique, later named Sun Breathing, and wanting to help others, Yoriichi tried to teach it to the other Demon Slayers. But no one could match his' exactly. 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. Michikatsu himself had his own style after trying to learn Sun Breathing but was unsuccessful, instead settling on his Moon Breathing.

Soon, Yoriichi encounters Muzan Kibutsuji, the progenitor demon and mortal enemy of the Demon Slayers, accompanied by Tamayo. Upon meeting him, Yoriichi could feel his rampant malice and violent intent that he believed was like 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, shortly after saying that, Muzan performed long sweeps of his arms to outright kill Yoriichi, who 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. Instead, he turn 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 and witness 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 and for Muzan to renegerate again. In spite of his failure, 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 and how the man that ruined her life remained alive, angrily cursing him. Upon realizing the Kibutsuji curse failed due to Muzan being weakened so much, Tamayo was surprised. Yoriichi approaches her and she told him everything he needed to know, how Muzan is the first demon and it is unlikely he would ever meet him again. As thanks for her aid, he let her free. Soon after this, several Demon Slayers came to report Yoriichi that his brother had betrayed the organization and became a demon. Yoriichi took responsibility for this, as well as his inability to stop Muzan and for letting Tamayo escape. The other Demon Slayer demanded he kill himself as penance but the newly appointed Oyakata prevent him doing so.

After his banishment, Yoriichi decided he had much in 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 spoke of everything he had in mind, telling him his beginnings, meeting Uta and her tragic death, becoming a Demon Slayer, meeting the demons Muzan and Tamayo and letting them escape, Michikatsu's transformation and 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 told he was welcome at any time, Yoriichi gave Sumiyoshi his own Hanafuda earrings. Sumiyoshi and Suyako realized then this was going to be the last time he will ever see them. Overcome with emotion, Sumiyoshi called out to Yoriichi as he walked away, calling him to stop saying that he was a man of no worth. He reminded that he saved them and he voiced his promised to preserve the Sun Breathing's form through his descendants. Hearing this, Yoriichi turns and smiles brightly, thanking the Kamados and waving them farewell.

Despite the apparently limited lifespan of 25 years suffered by those who bore the Demon Slayer Mark, Yoriichi lived till his 80s but eventually went blind as he continued slaying demons. On the last day of his life, he would personally confront Michikatsu, now known as Kokushibo and Muzan's strongest lieutenant as Upper Rank One. Devastated at what his brother had become, he cried, disgusting Kokushibo that his younger brother now appeared before him old and mourned his decision. In spite of his love for him, Yoriichi knew he had to kill him and took a stance.

Kokushibo's disgust turned to fear upon seeing that Yoriichi still was the same capable and powerful warrior from before. He told his brother his moment of attack and soon struck, slicing through his neck without Kokushibo able to even draw his blade. Angered at seeing him surpass him once 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 the former gifted to him when they were both still children. Remorseful and mourning the brother he always envied, Kokushibo took the split flute and carried it with him at all times until his eventual death, unable to let go of his memories of Yoriichi.

During the present battle against Muzan, Tanjiro on the verge of death through Muzan's blood would eventually see Yoriichi's past memories and even though he died hundreds of years ago, wished that his spirit could find peace.

Synopsis
Based on the past memory Tanjiro had from his ancestor, Sumiyoshi and Yoriichi had some sort of relationship, if not a friendship, given that Yoriichi would frequently stop at Sumiyoshi's house for food and tea. It is also hinted by Sumiyoshi that Yoriichi saved his life at some point before the birth of his child. From this, it could be implied Yoriichi had no successor both as a swordsman and in a familial sense. He also seemed mournful, likely because of the fact that he was unable to save his family, defeat Muzan, or prevent his brother from turning into a Demon.