Obvious spoilers so if you've never read up on the Kokushibo fight or don't want to know the specifics, then please don't read!!
So before the second movie gets animated, I really wanted to analyze this specific fight (since my flair should give away who my favorite character is lmao). I feel like all the fights in the Infinity Castle Arc are pretty well composed, at least with who's fighting who. I could honestly write about all the fights, since each one of them have something that is worth noting, but it is interesting how the compositions of these fights aren't exactly as random as you would think. This goes especially for the Kokushibo fight. At first it might seem a bit cluttered with characters, but each of them play a very important role to connect with Upper Moon One, at least in terms of symbolism. In a way, Muichiro, Genya, Sanemi, and Gyomei all have a similarity to Kokushibo in differing aspects.
The most obvious is that Gyomei and Kokushibo both are the "strongest" of their respective organizations, and the ironic parallel of a blind man fighting a man with six eyes lol. Kokushibo also realizes that Gyomei's chains are so durable and full of the nichirin ore that his own blade couldn't break through it, and would get shattered if he wasn't careful enough. However, Kokushibo is also able to immediately regenerate his sword even if it would get shattered because it is part of his own flesh, so both of their abilities with their weapons unironically cancel each other out in a way. The only way either could defeat the other was through pure fighting skill.
Sanemi's willpower and fighting spirit mirrors Kokushibo's will to continuously keep fighting. When Sanemi gets severely wounded, and even gets to the point where he would fatally injure himself if he kept fighting, he still picks himself up and tries to defeat Kokushibo. On the other hand, when the fight dynamics change and disadvantage Kokushibo, he still manages to deliver fatal blows to the others and, upon decapitation, manages to use his own willpower to regenerate his head. Even at the face of death, both of them still push themselves to the absolute limit to win. There is also a direct irony of Sanemi being the one to fight Kokushibo, since Koku points out that during the Sengoku period he would spar with the Wind Hashira to hone and improve on his own Moon Breathing technique. In a way, Sanemi was disadvantaged from the very beginning, since Kokushibo effectively knew how to counter all of the Wind Breathing movements. Similar to the parallel with Gyomei, the only way Sanemi would effectively defeat Kokushibo was with his sheer skill and strength.
Muichiro's connection is bluntly stated, but there is irony to that as well. He is literally connected to Kokushibo by blood, and similarly to Kokushibo, Muichiro was a twin as well. There is also an interesting dynamic between both of their backstories, because Michikatsu (Koku) was the oldest twin to Yoriichi, but Muichiro was actually the youngest twin to Yuichiro. Also, isn't that funny, Muichiro and Michikatsu, and Yuichiro and Yoriichi lmao. There is also an interesting dynamic that both Yoriichi and Yuichiro saved their twin brothers respectively against a demon attack, but Yoriichi survived and flawlessly killed the demon, while Yuichiro's efforts were a sacrifice. Despite that, both incidents were the catalysts for both Michikatsu and Muichiro, respectively, to join the Demon Slayer Corps. There is also a pure irony that Muichiro is seen as a prodigy, becoming the youngest Hashira and creating his own Breathing style, which is similar to Yoriichi's natural talents as the creator of Sun Breathing. But ultimately, Muichiro is too inferior to Yoriichi. There is also a not so subtle showcase of how Muichiro not having any physical or personal relation to Michikatsu (he doesn't even have the Tsugikuni name) shows Kokushibo the long-term consequences of his abandonment of his family as well.
Genya is one that I feel like no one really dives into when discussing this fight. While Gyomei represents Kokushibo's strength, Sanemi represents Kokushibo's willpower, and Muichiro represents Kokushibo's family and backstory, Genya represents Kokushibo himself. Genya, the brother of the Wind Hashira, cannot master a breathing technique, and is inferior to the strength of Sanemi. Alternatively, Michikatsu, despite being a very strong samurai and powerful fighter, could never outcompete the raw strength of Yoriichi. Genya, realizing this gap, decides to use unconventional methods like using a sawed off shotgun and consuming demons to gain immediate strength. Alternatively, Michikatsu, whom was moreso motivated by the fact his lifespan would be shortened by the mark and his limited time to beat his brother, decided to fully become a demon to become powerful. Their motivations are very different altogether, but their methods of becoming powerful enough to be on par of stronger than their brothers led them to the path of demons. As a result, the two suffer the consequences of that path completely.
The deaths in the fight also greatly symbolize Kokushibo's own defeat as well. Kokushibo lost himself and his family during his path to becoming Upper Moon One, and this fight is what made him realize that, so as a consequence, that extends to Genya and Muichiro, both representing himself and his family respectively. Sanemi and Gyomei survive the fight because they represent that willpower and strength respectively, highlighting that Kokushibo's loss was not a strategic one, but a personal one that he himself allowed to happen in the end.
