The Upper Ranks of the Demon Slayer in the Twelve Kizuki is still somewhat mysterious, but none more than the highest-ranked demon of them all, Kokushibo.
Demon Slayer Season 3 opens with a surprise - not by checking in with Tanjiro and the other Slayers, but by starting with the higher-ranked Twelve Kizuki, and finally introducing the most powerful demon besides Muzan himself, Kokushibo.
The Twelve Kizuki are the highest-ranking demons in Demon Slayer, and Muzan hand-picked them to be his enforcers. A few Kizuki are battled and defeated, including one of the higher ranks - hence the reason for grouping the remaining higher ranks together. The meeting mostly follows Akaza, a third-order demon as he arrives and meets the Others, who are quite an odd group. Kokushibo's existence is announced before it can properly appear, with only the first-ranked demon intervening as the bickering and infighting between the higher ranks grows considerably larger.
Kokushibo is Demon Slayer's Second Most Powerful Villain
Kokushibo gets a small chance to show his strength here as he progresses between Akaza and the second-class Doma, seen only briefly at the end of season two. Akaza's power was on display in Demon Slayer's Mugen Train, where he battled Kyojuro Rengoku, The Flame Hashira, and defeated him with relative ease. However, when Akaza raised his hand against Doma, his arm was cut off by the upper-rank Kokuchibu without giving him a chance to fight back. While healing from such an injury is trivial for these powerful demons, it does highlight the striking difference that exists between a Rank 3 and a Rank 1, with Doma saying that Akaza will never be able to defeat Doma or Kokochibo no matter how hard he tries. tries to improve.
Kokushibo is an interesting character, and he remains very much a mystery even after his first screen appearance. He seems to stick by the rules, which may be part of the reason why Muzan prefers him. Having someone like Kokushibo at the top helps keep the entire organization in line, after all. He shows some respect for the strength of his fellow demons in the higher ranks but demands order above all else. While it's not yet clear what (if any) advantages they offer, Kokushibo's most noteworthy facial feature in Demon Slayer is his three pairs of eyes, which make him instantly recognizable. Kokushibo's choice of clothing also differs from that of other higher-ranking Kizuki, as he wears samurai-like clothing, while the others wear very little (or in the case of the Doma, oddly modern clothing). Kokoshibo also bears a disturbing resemblance to a man seen in Tanjirou's dream, one who wore Hanafuda earrings like Tanjirou...
Kokushibo's Demon Slayer debut might have been brief, but it was enough to trigger the true power of the highest-ranking Demon Slayer in Muzan's army. While Tanjirou and the other Demon Slayers have made some progress, Kokuchibo's introduction to Demon Slayer Season 3 proves that they still have a long way to go before they even hope to defeat Muzan and his Upper ranks forever.