Kokushibo

Kokushibo is reserved, silent, and aloof, maintaining an aura of unnerving tranquility and mystery that compliments his position as Upper Rank One. He rarely speaks; when he does, he talks in a slow and emphatic manner that gives more gravitas and authority to his words. He is an adamant rule-follower, punctual, and he shows a great deal of respect to the hierarchy of the Twelve Kizuki, as seen at the Upper Rank gathering.[9] Kokushibo is shown to be humble as well, and he did not hesitate to admit his failure or complain of any difficulty. He displays a very strong sense of loyalty towards Muzan Kibutsuji, with most of his actions being done with the sole purpose of fulfilling Muzan's goals. Despite exhibiting outward reservation, dignity, and humility, there is a cold and unforgiving side to his personality; his words can be extremely harsh, especially when reprimanding individuals. His threats come off as nothing short of cruel and overwhelmingly serious, and he demands absolute obedience.

Kokushibo is shown to be genuinely delighted when the opponents he faced challenged him; such as helping dress Muichiro Tokito's amputated hand over his talented swordsmanship, praising Gyomei Himejima for his unparalleled human abilities and sparing Akaza because he enjoyed the challenge of battling him. However, due to this, he also felt irritation after he realized Akaza had died and he pities Gyomei for his impending death from unlocking his Demon Slayer Marks.

He possesses a complex relationship with his human past. He shows something of warmth towards his descendant, Muichiro, commending his skills and resolve, and being moved to convert Muichiro into a demon to spare his life. When their battle comes to an end Kokushibo displays his regret at having struck down his descendants.[10] At the same time, he refers to Muichiro and his abilities as the natural result of his "cells" being passed down.

In his duel against Gyomei, the demon's banter suggests a fear of death. When Gyomei's marks appear, Kokushibo pre-emptively bemoans the loss of a talented fighter and he urged him to become a demon to continue honing his skills. He seemed surprised when Gyomei vehemently rejected his offer and called his mentality pathetic.