Gyokko is a member of the Twelve Kizuki, one of the highest-ranking demons in the Demon Slayer Corps.
He holds the rank of Upper Moon Five (Kizuki Go) in the anime and manga series Demon Slayer: Kimetsu no Yaiba.
The Upper Moons are the upper half of the Twelve Kizuki, ranked from Upper Moon One to Upper Moon Six.
Gyokko is one of the major antagonists in the series and is known for his monstrous appearance and sadistic personality.
How many Hashira has Gyokko killed?
Gyokko, the Upper Moon Five demon from the anime and manga series Demon Slayer: Kimetsu no Yaiba, killed around 15 Hashiras (also known as the Pillars) before coming face to face with Muichiro Tokito, the Mist Pillar.
However, the names of the Hashira whom Gyokko has killed are unavailable. If it shows in Demon Slyer, we will be going to update this portion of the article.

Who will fight Upper Moon 5?
In the anime and manga series Demon Slayer: Kimetsu no Yaiba, the Upper Moon Five demon is a character named Gyokko.
Gyokko is one of the primary antagonists in the Sword Smith Village arc, along with Hantengu.
Gyokko came across the Mist Pillar Muichiro Tokito during the Sword Smith village arc; Muichiro confronted Gyokko for killing one of the swordsmiths, because of which a fight broke out between the two, and Muichiro using his Third Form: Scattering Mist Splash managed to cut Gyokko’s head only to learn that the body had been a puppet.
Then Gyokko started having fun with the mist pillar and showed him the artwork he had developed after eating his latest victims.
Gyokko moved from pot to pot and used his blood demon art Water Basin of Hell which wounded Muichiro.
But finally, after recovering, he used the Breath of Mist. Second Form: Layered Mist, Muichiro come out of the bubble trap.
Gyokko was shocked to see that Muichiro survived, so he transformed into his true form, and Muichiro then used the seventh form breath of Mist: Haze and covered the area with mist.
Then he used her silent steps to move around Gyokko and decapitated him savagely, just using his strength.
Kny! WARNING SPOILER
Roasting battle Mui x Gyokko coming soon 😎🤣
— KNYFESS 👹👺 CEK 📌 (@onifess) March 2, 2023
Why did Gyokko become a demon?
Gyokko’s original name before transforming into a demon was Managi, and Managi used to live alone on the outskirts of the fisherman’s village.
When a flood hit the village, his village was drowned, and his parents died as well; Managi, from his childhood, was an abnormal child who used to love collecting dead fish, and when his parents died due to the drowning incident, he was fascinated to see their bodies.
Most of the children in the village used to make fun of him and tease him because of his abnormal behaviour and his addiction to collecting dead fish.
Because of this, one of the children made fun of him, and due to this enraged, Managi killed him.
Due to this, the child’s parents stabbed Managi.
They left him to die, and eventually, Demon King Muzan Kibutsuji found him and was fascinated by the fact that a child had killed another child.
Because of this, he gave his demon blood to Managi and transformed him into Gyokko.
Gyokko became Upper Demon Five and started killing people to grow stronger in powers.
For a detailed backstory of Gyokko, you can go through the following link of the article.