He possesses a complex relationship with his human past. It is revealed that, as a human, Kokushibo, then Michikatsu, pitied Yoriichi during the period of time when he was mute, viewing him as a meek and callow boy dependent on his mother. He gifted him a flute to use when he needed his brother, and smiled at him despite being bruised from his father's beatings. However, he later harbored an immense sense of envy towards his younger twin brother for his natural talent and incredible abilities. These feelings of jealousy and contempt only became stronger upon seeing his brother become a peerless warrior of unmatched caliber among even the Demon Slayer Corps, with none of the Hashira coming close to his level of strength. This fostered a drive to become as strong or surpass his brother, a sentiment so strong that he abandoned his family to pursue becoming a Demon Slayer, and later, into a demon. His envy peaks when he discovers that Yoriichi is still alive and in old age, completely surpassing the curse of the Demon Slayer Marks that killed anyone that awakened them before they turned 25. Even centuries later, Yoriichi's immunity to the curse haunted Kokushibo, and the usually phlegmatic demon would become rattled when Gyomei inadvertently reminded him of Yoriichi by accusing him of lying about the curse having no exceptions.However, despite this immense jealousy, spite, and outright hatred he harbors for Yoriichi, he still deeply cares for his brother, as seen when he was touched by Yoriichi treasuring the handmade flute he had made for him as a child; Kokushibo sheds tears over his brother's death and even goes as far as keeping the flute itself for the following centuries as a memento. Kokushibo's fear of defeat stems from his inferiority complex and desire for strength. This fear causes him to become increasingly aggressive and desperate in battle, relying on his demon powers, and even killing and dismembering Muichiro despite his earlier to desire to turn him into a demon. However, as he faces off against the Hashira, Kokushibo realizes the heavy cost of his pursuit of strength. He becomes a grotesque monster, far from his idealized vision of becoming the strongest samurai like his brother, highlighting how his ambitions and resentment have twisted him. In his final moments, he is filled with sorrow and rage, lamenting his life choices upon seeing he hasn't achieved his goals and questioning if the path he chose was truly the right one. He realizes that his desire for a legacy had been for naught and he had ended up accomplishing nothing in his centuries of existence. As he disintegrates, Kokushibo angrily asks his deceased brother why he was even born, expressing his frustration at not achieving his desires. In the end, He highlights that he just wanted to become as strong and honoured like his younger twin brother. " I could never grab hold of anything. Anything at all. I abandoned my home. I abandoned my wife and children. I abandoned my humanity. I cut down my descendants and abandoned being a samurai. But even all that wasn't enough? You said that those who master their paths all reach the same place. But I never did. I could not see the same world that you did. Why could I not leave anything behind? Why could I not become someone known? Why are we so different? Why in the world was I ever born? Tell me... Yoriichi."