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 flew into a rage, and 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. Until he was seven, Yoriichi was thought deaf as he never spoke and was constantly found clinging to his sickly mother's left side (it is later revealed that he sensed her illness weakening her left side and was helping to support her while she walked). 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, at age 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 Michikatsu. 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 and had prepared to leave the night she died in order to protect his brother from being stripped of his title as successor and sent to the temple in Yoriichi's place. However, this caused Michikatsu to develop an intense hatred of Yoriichi, who he believed to be a genius and was extremely envious of him.

Even though he was to become a monk at a temple, Yoriichi did not do so. Instead he ran away, wanting to live freely and presumably to ensure Michikatsu position to succeed their family. 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 his blade back up and train as a Demon Slayer.

Eventually, Yoriichi and Michikatsu went on to become incredibly strong Demon Slayers, developing their own Breath Styles, Yoriichi's being the Sun Breathing and Michikatsu's being the Moon Breathing. Yoriichi went on to teach many people of the breathing techniques he used and of the Sun Breathing, however, since no one could reach his strength, Yoriichi instead trained his apprentices individual altered breath style training to better suit their individual strengths and weakness, eventually causing different derivatives to emerge from the Sun Breathing. Once the two gained their Demon Slayer Marks, Yoriichi expressed his plan to have a successor, despite his brother believing none could rise to their potential. Eventually, the two would part ways and Yoriichi would continue becoming stronger and hunting demons.

At some point Yoriichi encountered 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 natural talent to defeat Muzan once and for all. Muzan stated to Yoriichi that he had lost interest in swordsmen who employ breathing techniques as had encountered and slaughtered many of them. As soon as he said that, Muzan performed long sweeps trying to kill Yoriichi who instantly realized any of those attacks would be fatal and felt fear for the first time. Thinking he was born to kill this demon, Yoriichi fought Muzan and was able to discern Muzan's assemblage of seven hearts and five brains spread on his body, and was successfully able to cut through all of them with his Thirteenth Form.

Muzan was left in disbelief as he was unable to regenerate while struggling to keep his head attached. Yoriichi noticed Tamayo was hopeful for Muzan's destruction and the swordsman asked Muzan what the value of life was to him. Muzan didn't respond but instead grinded his teeth with his skin turning red from anger, and suddenly he bursted into 1,800 lumps of flesh of which Yoriichi cut 1,500 in mere seconds. He correctly deduced the remaining lumps must have reformed to regenerate somewhere else.

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. Yoriichi calmed Tamayo who proceeded to tell him that Muzan would likely avoid facing Yoriichi again and how she was glad to have been freed of Muzan's control due to being weakened. Yoriichi requested for Tamayo's help to which she reluctantly agreed. Soon after some Demon Slayers report to Yoriichi that his brother had betrayed the organization and become the demon Kokushibo. This along with Yoriichi's failure in killing Muzan infuriated many who wanted him to step down. The new head of the demon slayers, however, disagreed with this, although Yoriichi felt guilty for placing a huge burden on a six year old boy.

Despite the apparently limited lifespan of 25 years suffered by those who bore the Demon Slayer Mark, Yoriichi somehow lived for many more decades, but eventually went blind as he continued slaying demons. Later on in his 80s, he would personally confront his brother, now Muzan's strongest lieutenant as Upper Rank 1. Devastated at what his brother had become and trying to free him from his current fate, Yoriichi tried killing his brother, but only slightly sliced Kokushibo's neck. Although Kokushibo feared the next blow would kill him, Yoriichi unfortunately passed away soon after. 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.