Warning: Spoilers for Demon Slayer: The Swordsmith Village arc, Episode 1Season three of Demon Slayer opened with a surprise--not by checking in with Tanjiro and the other slayers, but by starting with the upper-ranked Kizuki, finally introducing the strongest demon besides Muzan himself, Kokushibo.
The Twelve Kizuki are the highest-ranked demons in Demon Slayer, hand-picked by Muzan to be his enforcers. A few Kizuki have been battled and defeated, including one of the upper ranks--hence the reason for gathering the remaining upper ranks together. The meeting mostly follows Akaza, the Upper Rank 3 demon as he arrives and meets the others, who are quite the weird bunch. Kokushibo's presence is made known before he properly appears, with the top-ranked demon only stepping in as the bickering and infighting among the Upper Ranks grew to be too much.
Kokushibo is Demon Slayer's Second Most Powerful Villain
Kokushibo gets a small opportunity to show off his power here as he steps in between Akaza and the rank 2 Doma, who was only briefly seen in the season 2 finale. Akaza's power was on display in Demon Slayer's Mugen Train film, where he battled Kyojuro Rengoku, the Flame Hashira, and defeated him with relative ease. Yet, when Akaza raised a hand against Doma, Kokushibo was sliced off the Upper-Rank demon's arm without so much as giving him a chance to react. While healing from such an injury is trivial for these powerful demons, it goes to highlight what an incredible difference there is between even rank 3 and rank 1, with Doma saying that Akaza will never be able to best himself or Kokushibo no matter how hard he tries to improve.
Kokushibo is an interesting figure, and he very much remains a mystery even after his first on-screen appearance. He seems to be a stickler for rules, which may be part of why he's a favorite of Muzan's. Having someone like Kokushibo at the top helps to keep the entire organization in line, after all. He does show some respect for the strength of his fellow demons in the Upper Ranks, but demands order above all. While it's not yet clear what (if any) advantages they provide, Kokushibo's most noteworthy facial feature in Demon Slayer are his three pairs of eyes, making him instantly recognizable. Kokushibo's choice of clothing is also pretty different from the other Upper Rank Kizuki, as he dresses in a manner similar to a samurai, while the others wear very little (or in Doma's case, oddly modern clothes). Kokushibo also bears a disturbing resemblance to a man seen in Tanjiro's dream, one who was wearing hanafuda earrings like Tanjiro's...
Kokushibo's first appearance in Demon Slayer may have been kept brief, but it was enough to tease the true power of the top-ranked demon in Muzan's army. While Tanjiro and the other Demon Slayers have made some progress, Kokushibo's introduction in Demon Slayer season 3 proves they've still got a long way to go before they can even hope to defeat Muzan and his Upper Ranks for good.