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 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 showing worry that he might have died in battle. 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. Gyokko was then greeted by Upper Two, Doma, who complimented him for a new pot and mentioned that the last one he was gifted by him now is being used as morbid decor, which earned the potted demon's approval. 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 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 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 advising the demons to fight harder. As the meeting concluded with Muzan's presence close off, Gyokko ranted about being joined on a mission he had discovered by himself, before being asked by Doma about what he knows. Gyokko refuses to answer, but when Doma pushed the question again, the demon witnesses his superior Akaza's act of slicing Doma's head for attempting to join a mission without Muzan's orders. 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, 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. When Muichiro Tokito and the two villagers come upon the shed looking for a new sword, Gyokko ambushes them from a ceramic pot. He introduces himself and asks if Muichiro would like to see his art. He then presents "Death Throes of the Smiths" a morbid piece consisting of multiple swordsmiths' bodies stitched together with swords sticking out of them. Once attacked by Muichiro, he retreats inside his ceramic pot and reveals he can transport himself from pot to pot at will. He explains that the pots are his pride and artwork, and becomes very upset and angered when they are damaged. He then summons many fish from a pot and uses Blood Demon Art: Thousand Needle Fish Kill to attack Muichiro, who lets himself be hit. Gyokko wonders what art he should make for Muichiro, but is rebuked and told he is worthless by Tokito. Upset again, he traps Muichiro with his Water Prison Pot technique, commenting that death by suffocation is "stylish and beautiful."

While Muichiro struggles to escape the Water Prison Pot, Gyokko advanced upon the shed. He enters and sees Hotaru Haganezuka working on the blade extracted from the training doll. Hotaru doesn't notice Gyokko's entrance behind him, enraging Gyokko at the prospect of another artist more concentrated on his craft than he. He summons a fish to attack Hotaru, breaking the sword-smith's mask but not his concentration. Hotaru's dedication to the blade incentives Gyokko to force him to break his concentration rather than straight-up murdering him. Muichiro escapes from the Water Prison Pot with help from Kotetsu, surprising Gyokko and causing him to unleash his Octopus Vase Hell technique. Muichiro's Sea of Clouds and Haze technique nicks Gyokko, and he ducks back into his pot, reappearing in a different pot. He argues with Muichiro about the beauty of his pots, becoming enraged after an insult to their balance, and unleashes his Ten Thousand Gliding Slime-Fish technique. After Muichiro deflects two of his attacks, Gyokko reveals his true form and becomes more serious about killing him. Gyokko's true form has the ability to turn anything he punches into fish, and has heightened speed and strength. Muichiro uses Obscuring Clouds to decapitate Gyokko, much to his disbelief.