Kokushibo is a reserved, silent, and aloof demon, maintaining an aura of unnerving tranquility and mystery. He rarely speaks; when he does, his words carry gravitas and authority. He is a rule-follower, respectful of the Twelve Kizuki hierarchy, and humble in admitting failures. He demonstrates unwavering loyalty to Muzan Kibutsuji. While reserved, he has a cold and unforgiving side, and his reprimands can be harsh. He is genuinely delighted by strong opponents, praising their abilities. He has a complex relationship with his human past, showing warmth towards his descendant Muichiro, but also regret for his actions. His immense jealousy of his brother Yoriichi fueled his ambition and transformation into a demon. Despite his jealousy, he deeply cared for Yoriichi, as evidenced by his keeping a treasured flute Yoriichi made. His fear of defeat and desire for strength led him down a path of regret and sorrow. As the Upper Rank One, Kokushibo is extraordinarily powerful, possessing immense strength, speed, durability, and regeneration abilities. He is a master swordsman, utilizing Moon Breathing and a flesh katana that he can regenerate. He can access the Transparent World, allowing him to predict opponents' movements. His defeat required the combined efforts of multiple powerful Demon Slayers.