Demon Slayer is famed for being a combat shonen anime with a heart. Many of these characters are sympathetic and engaging because they each have a strong emotional core, from Tanjiro's protective feelings toward Nezuko to Kyojuro Rengoku taking his mother's kind words to heart as he died. Even the demons often get tragic and heartfelt backstories, but not all of them do -- or should.
Several demons are intriguing because of their buried-but-not-forgotten human side, such as Rui and even the insidious Daki/Gyutaro pair. Now, in Demon Slayer's third season, the noxious Gyokko died without a sob story, and that made him feel far more alien and sinister -- if a bit shallow.
Gyokko Felt Alien as the Mysterious Upper Moon 5
Demon Slayer shines when it depicts its demonic villains as beings who lost their humanity to become creatures of the night, and this often plays out when demons like Rui and even the hand demon think back on their human days as they die. The main exception is the cold-blooded Muzan Kibutsuji, who was clearly never meant to be sympathetic, and now Gyokko, the fish and octopus-themed Upper Moon 5, joins that new trend.
This comes at a cost, with Gyokko feeling oddly shallow without any backstory or personal relationships to any humans or demons, but it does establish him as a truly alien threat who has no humanity whatsoever. Gyokko isn't a victim turned monster who secretly wants love -- he truly is just a thing -- a nightmarish creature who was only ever known as a grotesque demon. Even a shonen series like Demon Slayer, which relies on its strong emotional core to tell a story, needs a few villains who are true demons in every sense. There is no chance to redeem Gyokko or appreciate why he resorted to demonic status to realize his dreams. He is a cruel, unknowable alien entity, and he dies as one too, free of any tear-jerker flashbacks.
All this gives Gyokko a horror monster vibe, as he is a mysterious creature defined by the terrifying threat he poses to humanity, not his human side. Many Japanese horror stories, such as Junji Ito's works, rely on this trope, presenting the villainous being or monster as an unknowable thing that scares and disturbs the heroes by being so mysterious. Intellectually, such beings frighten and upset viewers for all the right reasons, defying humanity's need for sympathetic emotions and rational thought from other beings. Gyokko is a thing and nothing more, mocking Demon Slayer fans and characters alike with a nature that refuses to be rationalized or understood.
Demon Slayer's Upper Moons Should Feel More Alien, Like Gyokko
Certain members of the Upper Moons aren't just more powerful than the Lower Moons -- they also feel more alien, being far more than humanoid creatures with strange eyes and horns. Gyokko is the best example, being a many-armed creature with mouths for eyes and an eyeball mouth, and Hantengu loosely follows that trend with his bird-like features. Visually, this emphasizes the Upper Moons' vast distance from their former human lives, and it may deeply unsettle heroes like Tanjiro and Genya. These demons are the strongest and most gifted of all, and being unknowable, horror-style things makes them truly feel like fantasy supervillains and not just rogues with special powers.
Gyokko's utterly alien nature also represents his lack of personal vulnerabilities. Weaker, non-Moon demons look and dress like human beings for the most part, and they often have personal vulnerabilities to match. Even the Lower Moon Four, Mukago, looked fairly human and was frightened and insecure when Muzan deemed the Lower Moons unfit to serve him any longer. Meanwhile, Gyokko both looks and acts totally inhuman -- a smug and arrogant creature who fixates on cruel "art" and toys with his victims before killing them.
Perhaps Kokushibo, Doma and Akaza will do something similar when and if they assume final forms that wildly differentiate themselves from the humans they once were, and acting and looking this way will also help end the "demon with a sob story" trend that has almost become a running gag by this point. Ideally, Gyutaro and Daki will be the last Upper Moons to do this, and the rest will menace the demon slayers as mysterious, alien beings who are impossible to sympathize with or understand, even in their dying moments.