Gyokko/Synopsis

Human Life
Managi, as he was known as while still human, was born and lived in a village at the edge of a coastline, the village known for its fishing. When he was young, his parents went out on a fishing trip and never returned, as they would end up drowning far out at sea. After some time, their mangled corpses washed up and appeared on the village's shore. Upon seeing his parent's bodies, Managi developed a grotesque fascination with corpses.

Villagers would often see him obsessing over fish carcasses he found, leading the entire community to ostracize him and keep their distance, believing Managi had gone insane. Eventually, a child found Managi and teased him for his unusual behaviour, leading Managi to kill the child and stuff the body in a pot used for fishing. When the child's parents discovered them dead and Managi had been responsible, they stabbed him with a pronged fishing spear and left him to die on the spot.

For twelve hours, Managi would barely stay alive as his wounds ebbed his life away, up until he was discovered by Muzan Kibutsuji and transformed into a demon.

Demon Life
As a demon, Managi took on the demon name Gyokko. Since his near-death experience, Gyokko developed a strong hatred for all living things, embracing his new life as a demon and worshipping Muzan. He would primarily eat the flesh of children. His strange obsession with fish and corpses would reach a new twisted peak when he began to modify his body to look like a fish, and he developed a penchant for abducting people through pots of his creation. Once he captures his unfortunate victims, Gyokko would turn them into "art", using their bodies to make living statues to fit his grotesque artistic view. Often time, these victims would stay alive despite being their bodies horribly mangled.

In time, Gyokko would eventually attain the rank of Upper Five among the Twelve Kizuki.

Swordsmith Village Arc
Being summoned to a gathering of the Upper Ranks, Gyokko greets the recently arrived Akaza, asking him how he had been after 90 years of not seeing him and feigning worry that he might have died in battle, which annoyed the higher ranked demon. He was then corrected by Hantengu, Upper Rank Four, and told that it has been 113 years since the last summoning of the Upper Ranks, terrified that Gyokko had "forgotten how to count" the time they've been apart. Gyokko was then greeted by Upper Two, Doma, who complimented him for a new pots and mentioned that the last one he was gifted by him now is being used as morbid décor, which earned the potted demon's approval, although he commented that he had not intended that to be its use. He then witnessed Doma get attacked by Akaza for touching him and heard as Upper One, Kokushibo, announced Muzan's arrival to the castle.

During the meeting, Gyokko sat quietly as Muzan informed them of the death of Upper Rank Six, Gyutaro. The Demon King admonishes them for failing to kill both the Ubuyashiki family and being unable to find the Blue Spider Lily. When Muzan angrily stated that he does not know why they exist, Gyokko speaks up and informs his master of new information he possessed. Suddenly, Upper Five found himself as a severed head in Muzan's hand, decapitated by the Demon King and being unable to comprehend his speed. As Muzan read his mind and remarked about his detest towards all forms of change, Gyokko agreed while simultaneously enjoying being touched by his master. Upon seeing his information, Muzan demanded that Gyokko never give him unsourced information and assigned him on a joint mission with Hantengu to infiltrate the Swordsmith Village, before dropping the demon's head and ordering the demons to fight harder. As the meeting concluded with Muzan's presence close off, Gyokko ranted about having to be joined on a mission he had discovered by himself, before being asked by Doma about what he knows. Gyokko hesitates answering, possibly fearing punishment for Doma's rogueness, but when Doma pushed the question again, the demon witnesses his superior Akaza's act of slicing the top of Doma's head for attempting to join a mission without Muzan's orders, which he seemed rather perturbed by. Gyokko then saw as Kokushibo sliced off Upper Three's hand for insubordination, leading to a tense stand off that ends with both demons leaving the Infinity Castle. Shortly after, using the hands growing from his head to run away from Doma, Gyokko commanded the Biwa Demon to transport himself and Hantengu to the Swordsmith Village. At the Swordsmith Village, Gyokko devoured an unsuspecting villager but immediately spat him back out, stating that he "wasn't edible". He then noted that destroying the swordsmiths' village would severely weaken the Demon Slayers.Gyokko then unleashed several fish-demons made up of his Blood Demon Art, setting them loose to wreak havoc on the village. Unbeknownst to him, his attempt at destroying it would be foiled by the unexpected arrival of Mitsuri Kanroji.

He then hid in a shrub close to a work shed and waited for another unsuspecting victim to kill. He was surprised to see that his would-be victim had discerned his presence and emerged out of his hiding spot to find the Mist Hashira Muichiro Tokito, accompanied by two swordsmiths. Seeing his chance, Gyokko revealed to the three a piece he had made and displayed a collection of swordsmiths mangled together that he had dubbed "Death Throes of the Smiths". The two swordsmiths react with horror at seeing familiar faces tortured, while the Hashira immediately attempts to slash his neck in retaliation. Once attacked by Muichiro, he retreats inside his ceramic pot and reappears in a new one, revealing that he can transport himself from pot to pot at will. He adds that his pots are his pride and artwork, and grows angered when they are damaged by the Mist Hashira. He summons goldfish from a pot in his hand and uses Blood Demon Art: Thousand Needle Fish Kill to attack Muichiro, who lets himself be hit to protect the two swordsmiths. Gyokko wonders what art he should make for Muichiro, but is rebuked and told he is worthless by Muichiro, who attempts another slash at his neck. Enraged again, he traps Muichiro with Blood Demon Art: Water Prison Pot, commenting that death by suffocation is "stylish and beautiful." He then abandons the Hashira to die and makes his way to the work shed. While Muichiro struggles to escape the Water Prison Pot, Gyokko enters and sees the swordsmith Hotaru Haganezuka working on the blade extracted from the training doll. One of the swordsmiths with Muichiro, Kozo Kanamori, attempts to stop him but is attacked and injured by The Upper Rank. Hotaru doesn't notice Gyokko's entrance behind him, enraging the demon at the prospect of another artist more concentrated on his craft than he. Despite attacking Hotaru several times, even causing horrific wounds to the smith's body and destroying his mask, he continues to remain focused on polishing a blade he is working on. Hotaru's dedication to the blade incentives Gyokko to force him to break his concentration rather than murdering him outright. Eventually, even after enduring considerable physical damage, Hotaru continues to ignore him and the enraged Gyokko decides on killing Kozo to ensure a reaction out of him. Suddenly, he senses something near him and ducks back into his vase to avoid Muichiro's unexpected slash. Seeing the Hashira confuses the demon, who is unsure how he had escaped his Water Prison Pot and notices marks on Muichiro's face, ones that Muzan had told him were similar to another Demon Slayer and strengthened him. As Muichiro attacks, Gyokko aims a pot and unleashes Octopus Vase Hell, forming giant rubbery tentacles that grab Muichiro and Kozo and destroy the work shed.

He quickly notices Hotaru knocked outside but continuing to diligently polish his blade, much to his ire. Deciding he will kill the swordsmith last, he turns to see Muichiro and Kozo trapped and squeezed by his tentacles, remarking that their rubbery structure makes them impervious to slashes. To his utter shock, the tentacles fall to pieces as Muichiro frees himself and Kozo, using a new Nichirin sword forged by the latter. Despite Gyokko's attempt to grab and crush him with his tentacles, Muichiro easily slices through them and aims a slash at his neck, forcing Gyokko to duck into his vase and reappearing up a tree. He taunts the Hashira by noting that he is still unable to match his speed at evading him, only to receive another surprise when Muichiro notes that he was able to land a cut on his neck. Demanding that Muichiro does not underestimate him, Gyokko blitzes to the ground and fires jets of water at the Mist Hashira, who is able to weave his way through the forest and evade them. The demon then summons more goldfish to riddle Muichiro with needles, only to discover they are unable to hit their mark with Muichiro moving too quickly for them. He summons more tentacles to grab his opponent, who leaps up and slashes the vases out of his hands, angering him greatly. At a standstill with with the Hashira, Gyokko and Muichiro trade insults over each other's appearance and hatred for the other, which ends with Muichiro deriding his pots as shoddy workmanship. Livid, the Upper Rank forms ten pots in his hands and uses Blood Demon Art: Ten Thousand Gliding Slime-Fish, launching a giant shoal of ravenous fish at the Hashira. Gyokko's confidence in his attack soon shatters when Muichiro, strengthened by his marks, manages to easily engage the fish at high speeds and slice them apart. Seeing the fish destroyed, Gyokko notes that they release a fine poison that can be easily absorbed through the skin and remains hopeful that Muichiro will die, only for the Demon Slayer to quickly deflect the toxic cloud and suddenly lunge at him. Gyokko quickly reads the attack and sheds his skin, reappearing amongst the trees in his true form. His pride in himself now restored, Gyokko taunts Muichiro with his confidence that he cannot be defeated in his new body, claiming that only three people have witnessed his actual appearance and his new scales and body make him more powerful than before.

Despite this, Muichiro only blankly stares at the Upper Rank, causing Gyokko to finally snap and angrily demand that Muichiro acknowledge him. As Muichiro claims he is not in the slightest surprised in his transformation, the demon rushes at him and attempts to punch the Hashira to death. Though Muichiro evades his frenzy and hides atop a tree, a portion of his uniform is turned into fish, as do the impact points of Gyokko's fist. Upper Five laughs at seeing his opponent on the defensive and remarks that anything he touches transforms into fish, asking if the now quiet Demon Slayer is trembling with fear. Muichiro, with a mocking grin, instead notes that the most powerful attacks are pointless when they fail to hit a target. Watching as Muichiro lands down to face him for the last time, Gyokko uses Blood Demon Art: Killer Fish Scales and uses his new body to encircle the Mist Hashira in an attempt to attack and touch him. As he moves about, Gyokko boasts of his new body's ability to defy the laws of nature with his unnatural movements and loudly boasts of his intentions to decapitate the Hashira and transform him into his art piece. Watching Muichiro seemingly stands still, the demon sees his chance and throws a punch to kill him. Unexpectedly, his fist connects with nothing as his opponent seemingly vanishes mid-strike. Gyokko sees Muichiro appear again and lunges for another attack, but fails to hit once again. Continuing to see his opponent appear and disappear around him, Gyokko envisions himself seemingly trapped within a thick cloud of mist. Muichiro, after effortlessly dodging his blows, approaches the utterly confused demon and asks if he had believed himself to be the only one who wasn't taking the battle seriously. His blade successfully slices through the Upper Rank's head, with Gyokko entirely unable to see him. Seeing the world around him upside down, Gyokko finally sees Muichiro and realizes he must quickly kill him and report to Muzan. To his surprise, his head bounces on the ground as the sobering realization of his decapitation creeps up on him, astounded to think he was defeated. Watching as Muichiro walks up to him and demands he does not reincarnate, an infuriated and insulted Gyokko attempts to grow his body back slowly and launches into a tirade, repeatedly boasting of his superiority over his human killer and how he is the perfect being. Muichiro, having had enough of his empty threats, finally silences Upper Rank Five by mincing the talking severed head into hundreds of pieces. Without the strength to regenerate anymore, Gyokko silently disintegrates, marking an inglorious end to the Upper Rank demon.

With Gyokko's death, and the subsequent demise of Hantengu, their mission to wipe out the Swordsmith Village had been an utter failure. However, Muzan did not even care about the loss, instead being more focused on Nezuko Kamado, who had unexpectedly gain the ability withstand sunlight, much to his joy. Interestingly, Muzan never replaced the Upper Rank Five position, and it was left vacant following Gyokko's death, unlike Upper Rank Six and Upper Rank Four, which was filled in by Kaigaku and Nakime respectively.