Unlike most characters in the story, Yoriichi actually had a serious love interest. He met Uta for the first time in a rice field, where she revealed her struggles and earned his respect. Yoriichi and Uta subsequently returned to her house, living together for over a decade.
Uta meant a great deal to Yoriichi, as he never married another woman after her, as he had no heirs. In addition, he would also carry a bag made from Uta's kimono with him, for the rest of his life to remember her by.
Yoriichi and Tanjiro are connected by way of being chosen to wield Sun Breathing, but Yoriichi died long after his wife and child had died, meaning he had no direct descendants.
I always had the idea that Yoriichi killed himself by holding his breath during his last attack because he couldn't bear to live knowing he killed his brother. He never weakened, he never slowed down, and he never lost his skill. The only reason Kokushibo survived that encounter was because Yoriichi wanted him to.
Tanjiro's connection to Yoriichi is not through blood relation, but through their similar appearances, which is due to the divine nature of Sun Breathing.
He shows something of warmth towards his descendant, Muichiro, commending his skills and resolve and being moved to offer Muichiro a chance to become a demon. When their battle comes to an end, Kokushibo displays his regret at having struck down his descendants.
Kokushibo absolutely still loved Yoriichi, despite how much he tried convincing himself that he didn't. He lied to himself for centuries to soften the pain, but he genuinely still loved his brother, just like his brother still loved him.
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.
However, Mitsuri passes away while fighting Muzan Kibutsuji in the Sunrise Countdown arc. She displays extraordinary strength, ripping off the Demon King's arm. Despite her power move, Muzan's attacks tear her flesh and limbs, and she succumbs to death during the battle.
Muichiro and Obanai gripped their blades with all their strength, so technically, they did activate at will. Yoriichi probably does the same thing and makes his blade red with gripping power, except he's so strong, he can do it however and whenever he wants to.
Kokushibo roars at the absolute top of his lungs in anger and frustration. As he roars, dozens of chaotic slashes release from his body, cutting Muichiro's lower half of his body clean off, guaranteeing his death, along with cutting Genya in half vertically.
Genya has some minor infatuation on Sakura Igarashi. He was very extremely jealous of Hikaru Ushijima for having his crush and later he might become suspicious about his business that Hikaru mentioned.
Enraged by the feeling of inferiority, he slashed Yoriichi's body in half, but inadvertently broke the flute he gave him as a child. Seeing that he still thought highly of him even after his transformation, Kokushibo couldn't help but shed a tear.
There is hostility between Kokushibo and Akaza. He doesn't get along with Akaza, since he sees him as a disgruntled individual who disrespects the Twelve Kizuki members who are of a higher ranking than him.
Kokushibo has always hate Yoriichi, because he was stronger than him, with all this jealousy he would never turn him into a demon because Yoriichi would maybe be stronger than Kokushibo.
It is revealed that Muichiro suffers from amnesia caused by his distressing past. Many of his personality traits line up with the symptoms of PTSD. It would explain his poor memory, difficulty concentrating, and hypervigilance.
He was arguably one of the closest to Muzan, who viewed him as somewhat of a business partner.In turn, Kokushibo became intensely loyal to Muzan, acting as an enforcer for the hierarchy he built among the Upper Ranks, as shown when he punished and threatened Akaza for attacking Doma.
