Yoriichi Tsugikuni/Relationships

Uta
Soon after he ran away from home Yoriichi encountered Uta in the mountains at a paddy, standing still all by herself holding a bucket in her hands. After a while, Yoriichi asked her what she was doing Uta replied that she was all alone now as her family had died due to a disease and felt lonely so she decided to take some tadpoles to her home but continued to stand still and as the sun began to set, she lowered her bucket and let the tadpoles go. Yoriichi asked her why she decided not to take them home with her and Uta responded that she felt bad about separating the tadpoles from their parents and siblings. Then the young boy offered to go with her instead and she agreed.

Uta was very talkative and contrasted well with Yoriichi's silent personality, she would talk all day and through her Yoriichi learned for the first time that the rest of the world saw things differently than him, discovering that being able to see through other living creatures was not something other people could do. As the pair lived together they grew quite close and whenever they went out Uta would always grab Yoriichi's hand very tightly, as if holding a kite whose string had been cut loose.

Ten years after they first met the two got married and in the final month of Uta's pregnancy, Yoriichi had to leave her at home as he went to fetch a midwife, however upon returning he found that his wife and unborn child had both been slaughtered in cold blood by a Demon. Yoriichi was devastated by this and spent a full 10 days embracing their corpses until a Demon Slayer passed by and advised him to bury them. Uta meant a great deal to Yoriichi and he likely never married another woman after her as he had no heirs, in addition to also carrying a bag likely made from Uta's kimono with him for the rest of his life.

Muzan Kibutsuji
In their initial confrontation with one another for the first time, though Yoriichi showed no fighting spirit, hatred, or killing intent, he viewed killing Muzan as the reason that he was born and the reason that he held such strength. But, despite succeeding in striking him in all his weak spots and cutting off his head using the thirteenth form of his Sun Breathing, Muzan managed to escape. It was both failing to kill Muzan and his brother Michikatsu turning into a demon that caused his banishment from the Demon Slayer Corps, which drove him to leave, but not before he left behind his earrings and Breath Style to Sumiyoshi.

Tamayo
Yoriichi met Tamayo at the same time as Muzan, first believing her to be one of his henchmen, but realized that it was incorrect after realizing that she was elated after witnessing that Muzan was close to death. After he escaped through scattering into pieces of flesh, Tamayo though hesitant at first agreed to assist Yoriichi in defeating him.

Michikatsu Tsugikuni
He was Yoriichi's elder twin brother who was born only a few moments before him. Due to being older, he was in line to be their father's successor while Yoriichi was was to be sent off to a monastery when he was old enough. Yoriichi would be all alone by himself in their house while Michikatsu received all the attention as the heir to their father. Yoriichi would always cling to their mother and his brother found this to be pitiful. One day Michikatsu made his brother a wooden flute, a gesture the young Yoriichi appreciated greatly and even smiled for the first time ever upon receiving this gift. He would keep this flute with himself until the day he died. Wanting to spend more time with his brother Yoriichi attempted to learn sword fighting alongside him and greatly outclassed Michikatsu's teacher, who his elder brother had barely even managed to strike throughout his training. When their mother suddenly died of an illness Michikatsu found out that she had been weak in her left side due to the sickness and deduced that Yoriichi always clinging to their mother was in fact to support her and this greatly troubled Michikatsu, who quickly became jealous of his younger brother's prowess.

Their father had been planning on making Yoriichi his heir after learning of his exquisite swordfigthing skills but the young boy knew his brother had been trained his entire life to be the heir and suddenly having it snatched away from him would be devastating. So Yoriichi decided to run away from home to avoid that from happening.

Yoriichi looked up to his brother and bore him no ill will, even after years of separation he rushed to save Michikatsu when his forces were battling demons to protect their home. Swiftly dealing with them all, Yoriichi turned around and apologised to his brother for being late and not being able to save any of his men. Michikatsu was greatly jealous of his younger brother and upon seeing him again those feelings reignited and he followed him into the Demon Slayer Corps. He attempted to learn Yoriichi's Sun Breathing and while he was able to emulate it better than anyone else due to his close relationship and similarity to Yoriichi, he was still unable to perform it properly, creating a new form of Breathing called Moon Breathing.

Confident in his and his brother's progress, Michikatsu once declared to Yoriichi that they were the greatest swordsmen of all time and that no one else would ever be able to rival them. Yoriichi disagreed and told his brother that they were but small droplets in the vast history of the world and that there would surely come a time where someone who surpassed them would appear and that they should feel secure with this knowledge. Still frustrated at his continued failure to overtake Yoriichi, Michikatsu's jealousy and his fear that death at a young age would rob him of the chance to grow even stronger spurred him into accepting Muzan Kibutsuji's offer to turn him into a demon when the man approached him. Michikatsu became Kokushibo and would eventually gain the position of Upper Moon 1. When Yoriichi learned of his brother's decision in the aftermath of his battle with Muzan, he was greatly saddened. This incident- alongside a number of other factors- led to Yoriichi's exile from the Demon Slayer Corps.

Many decades later, an elder and wizened Yoriichi encounters his brother one last time and prepares to battle him. He apologizes to Kokushibo with tears in his eyes before steeling himself. The brothers both attack each other but it is Yoriichi who gets the hit in, slicing his brother deeply through the neck, leaving him but one blow away from death. However, Yoriichi lowers his blade and it sinks into the ground, with him leaning on it as he dies before delivering the final strike. Angered at yet another failure to even land a hit on his brother, Kokushibo slices through Yoriichi's dead body in sheer rage. He sees an object fall out of his brother's kimono and upon closer inspection realizes that it is the flute he had made for him in their childhood. Kokushibo tears up at the gesture, saddened by it but unable to move past his inferiority to his brother. Regardless, the flute would remain with him until his own death centuries later.

Sumiyoshi
We are not told how Sumiyoshi and Yoriichi met, but based on the manga, we can see that Yoriichi and Sumiyoshi were good friends, though they did not see each other very often. After Yoriichi was banished from the Demon Hunters, he decided to visit Sumiyoshi, because he was the first person who came to mind when Yoriichi wanted to talk to someone about how difficult his life has become. He begins to explain the situation, as to where he let Muzan and Tamayo escape and how his brother became a demon and how he was banished from the Demon Slaying Corps, after which left Sumiyoshi speechless, with only tears streaming down his face. Then Sumiyoshi's daughter, Sumire, started to tug on Yoriichi's haori, to carry her. Seeing the joy on Sumire's face from being lifted up, made Yoriichi cry, when Suyako, Sumiyoshi's wife, returned from picking chestnuts. Later, Suyako asked Yoriichi to demonstrate all of the Sun Breathing forms, to which Sumiyoshi engraved it into his mind. Just before Yoriichi left, he gave Sumiyoshi his Hanafuda earrings, when Sumiyoshi realised he would never see Yoriichi again and as Yoriichi left, Sumiyoshi promised to keep the earrings and told Yoriichi to never think he never accomplished anything and that he would carry on Yoriichi's legacy, to which Yoriichi replied "Thank You", before disappearing to never be seen again by Sumiyoshi.