The sufferings of my past were lessons on what to do to others as revenge! If I don't cause misery equal to my own unhappiness, I'll never balance the scales! That's how it is when you're a Demon! And we kill anyone who tries to tell us otherwise.
Gyutaro was a tall man with a black-dotted, pale grayish complexion whose anatomy was somewhat strange in that his upper torso, arms and legs were all extremely muscular where his waist was incredibly skinny, his pelvis jutting out at an unnatural angle. He possessed thin, green and red eyes, their sclera bright orange, that were slanted drastically downwards at the sides, the kanji for "Upper" and "Six" carved across them. He had unruly black hair that cut sharply off to a brighter green around the crown of his head, worn messily styled with uneven bangs and several ahoges. Gyutaro has been noted several times to have looked very ugly.
Gyutaro wore no shirt, only sporting a pair of baggy blue pants and seven pieces of pattern red cloth to decorate his body, one large one around his neck and three smaller ones around each arm.
Due to the hard life he lived in near-constant poverty and the harsh tragedies he experienced as a human, Gyutaro portrayed himself to be cruel, petty and envious of anyone blessed with a better life than him, leading to him developing a habit of sarcastically complimenting a person's best traits and then trying to vindictively kill them afterwards, as he did to Tengen Uzui. Because of these hardships he faced as a human, Gyutaro eventually developed a personal belief that to take from those who have a better life than he had as a human is justified. A habit he seems to have is the penchant to inflict harm to himself when he is angry, frustrated or jealous as seen when he scratched himself hard enough to tear deep wounds into his own flesh when he grew jealous of Tengen.
Having lived over a hundred years and battled many Demon Slayers and Hashiras, Gyutaro has become somewhat arrogant and prideful in combat, developing the habit of underestimating his opponents and looking down on them as beneath him, as he did with the team of Demon Slayers dispatched to kill him by having his sister fight and deal with them first rather than himself.
Despite his negative traits, Gyutaro does possess a genuine, strong love for his younger sister, Daki. Having raised her himself due to his mother's mental instability, this familial love was so strong that, despite becoming a Demon and nearly forgetting his life as a human, he still retains a considerable attachment to her. Due to this, he has developed a tendency to occasionally spoil and allow her to do whatever she pleases, even if her actions go against his best interests. However, his love for her does not stop him from screaming and insulting her out of frustration.
Upon his death and his final thoughts, Gyutaro is shown to possess no regrets on the choices he made throughout his life—especially his choice of willingly becoming a Demon and his subsequent actions of killing and eating hundreds of people for hundreds of years—showing how misanthropic and bitter he had become due to his terrible and harsh circumstances as a human. However, he later states that his only true regret he holds in his life is his sister, Ume, showing that deep down, Gyutaro regrets that his negative influence through how he raised her was the cause for her growth into a selfish person. During his final moments, Gyutaro wonders if she could have experienced many other wonderful things and lived many different lives without having to walk on the path of becoming a Demon if he wasn't in her life. He genuinely wishes that she was born and raised by someone other than himself, completely shouldering the blame for all of her deplorable actions and showing just how much he loves her.
Later, even in death, Gyutaro is shown to have Daki's best interests at heart by telling her to go to the opposite direction where the light was better, showing that he wished Daki would have a fresh start to a new life without him being a bad influence on her.
Gyutaro was born into poverty within the lowest class of the Entertainment District. His mother would often beat and try to kill him since she could not afford to feed him, but her efforts were proven to be in vain when she died of syphilis.
Throughout his childhood, he was berated for his appearance, voice, and poor hygiene from the other residents of the district. When his sister, Ume, was born he gained a sense of pride and acted as her "collector" within the district.
When his sister turned 13 and she began to officially work for a brothel, he returned home from work to discover her tied up and burned to near death and left to die in a ditch because poked a samurai's eye out with a hairpin. He jumped into the ditch and began to panic, tearfully demanded that his sister be turned back to normal while cradling her body. As he embraces his dying sister, the same samurai whose eye was poked out, sneaks up behind Gyutaro and slashes his back in an attempt to kill him. He then overheard the samurai speaking to brothel manager who is revealed to be the one who asked to have Gyutaro killed. However, before the samurai is able to finish him off, he manages to jump out of the ditch and kill both the manager and the Samurai with his sickle.
Despite being injured, Gyutaro carried his sister Ume's dying body throughout the district in search of help, but was ultimately denied aid no matter where he went. He eventually succumbed to exhaustion, both he and Ume being on the verge of death. Before the end, they were then found by the then Upper Rank 6 Doma, who was searching the Entertainment Districts to devour the young women of the area. He then offered to "help" them by turning them into Demons, due to him being a "Nice Guy" and gave them a few drops of his blood to initiate their transformations. He then gleefully challenged them to become powerful enough to be chosen by "that man" and join the Twelve Kizuki themselves.
As a Demon he and his sister, who was now known as "Daki", continued to reside in the Entertainment District and continued to terrorize and feast on its inhabitants, with the siblings seemingly developing a taste of only eating from the district's finest and most beautiful Geisha and Oiran of the district. After an unknown number of years, the siblings were eventually noticed and acknowledged by "that man" and given the rank of Upper Rank 6 of the Twelve Kizuki that they then shared between themselves.
Somewhere along his long life as a Demon, Gyutaro successfully killed and ate at least thirteen Hashiras of the Demon Slayer Corps while his sister killed and ate seven.
After a tantrum-throwing Daki called out for him, Gyutaro emerged from her body, quickly taking her to another part of the room. He reattached her severed head, comforting her while doing so. Tengen attacked the pair, but Gyutaro quickly countered it, injuring the Hashira. He then complimented the Demon Slayer on stopping his attack.
The mysterious Demon further complimented Tengen's face, skin, flesh, and height, as well as his presumed popularity with women. Feeling envious of the Hashira, Gyutaro threatened him with a painful death. Hearing Daki's complaints about Nezuko Kamado's involvement, he promised to exact vengeance upon all those who had hurt her. Throwing his sickle-like weapons with great force, which returned to him shortly afterwards, Gyutaro was unable to injure Tengen or the bystanders he was protecting. He restated his envy of the Hashira, which only increased when the Demon Hunter revealed he had three wives. The mysterious Demon unleashed his Blood Demon Art, Splatter Sickle, a barrage of blood blades. His attack accomplished nothing, as Tengen escaped through a hole he made in the floor, which surprised the emaciated Demon. Gyutaro commanded his attack to turn, revealing its auto-tracking nature. Tengen deflected the attack again, throwing and then detonating several small bombs in Gyutaro's vicinity. Wrapped in his sister's sash, Gyutaro withstood the blast, smugly stating they are one.
The emaciated Demon informed his opponent he was different from the previous Hashira he had fought, once again expressing his envy at Tengen's talents. He remained silent as the Sound Hashira rejected his claims, who cited Gyomei Himejima and Muichiro Tokito as examples of amazing talent, but countered by demanding to know how his Blood Sickle's poison hadn't killed the Demon Hunter yet. Moments later, Gyutaro grinned, declaring Tengen's statement of poison resistance to be a bluff, and blocked the Hashira's twins blades with his dual sickles. Clashing briefly with his opponent, Gyutaro was furious at Tengen kicking his sister. He was then surprised as Tengen threw more bombs at him and his sister, which he managed to escape unscathed. He quickly deduced the true nature of the bombs, special gunpowder capable of hurting Demons, leaving him confident in their eventual victory, but was surprised as Tengen's subsequent sword strike extended. Marveling at the Hashira's grip strength, Gyutaro deflected the attack, but was left with a slight wound to his neck. He warned Tengen of his impending death, only to be surprised by the arrival of Inosuke Hashibira and Zenitsu Agatsuma.
The mysterious Demon reiterated his death threat, believing the arrival of the Hashira's subordinates to be insufficient. Tengen remained confident in their victory, having deduced that cutting off both of the siblings' heads off at the same time was the key to success, but Gyutaro assured him it wouldn't be easy to accomplish. He also mentioned how he and his sister had eaten numerous Hashiras, fifteen and seven, respectively. He then watched silently as Daki was taken onto the roof by Zenitsu, with Inosuke going to assist the towheaded youth in his fight, but remained confident that she would prevail. Utilizing his connection with his sibling, Gyutaro closed one of his eyes, and quickly attacked Tanjiro Kamado. He nearly struck a blow to the youth's neck, but was thwarted by Tengen at the last second causing him to only stab him in the chin mere inches away from the fatal strike, and proceeded to exchange a flurry of blows with his opponent. In the middle of this clash, sashes from his sister's conflict above them descended, prompting Gyutaro to call Tengen's claim of success to be a lie, as well as labeling his adversaries as incompetent for lack of movement control.
Overall Abilities: As the true holder of the Upper Rank 6 position and receiving more blood from Muzan, Gyutaro was an incredibly powerful demon, with he himself having the necessary power, experience and skill to easily combat both Tengen, the Sound Hashira of the Demon Slayer Corps and Tanjiro, a very talented fighter and overwhelm them both in battle. Gyutaro also later stated that he had successfully killed and eaten thirteen Hashira during his long life as a Demon, further showcasing his powers as an Upper Rank. Another testament to his power is the fact that despite being killed in battle, Gyutaro was still capable of severely injuring and maiming Tengen Uzui, the Sound Hashira and severely injuring and poisoning him, Tanjiro and Inosuke and would have likely killed them if not for Nezuko's own unique Blood Demon Art and if not for his own arrogance getting the better of him, he would have likely won and killed them all himself. His abilities are a more advanced version of his sisters's powers, as Daki can only manipulate her flesh while Gyutaro could manipulate more aspects of his bodily compositions, including his blood, flesh and bones.
Cell Manipulation: Gyutaro can also manipulate his own cells, shown when he can encroach and infect the swords of Tengen Uzui and Tanjiro Kamado with his flesh sickles in order to prevent them from pulling out their blades effectively keep them in place. This is also shown with Gyutaro possessing enough skill in this ability to manipulate his body at a cellular level and even merge his own body with the body of his sister in order to boost all of her powers.
Core Detachment: Gyutaro possess the ability to completely detach his "Core" from his own body, with it seemingly taking the form of his right eye that he then implants into his sister Daki's forehead, that then enhances and strengthens all of his sister's abilities and powers, specifically increasing her kinetic vision to the point that she could easily keep up with the high speed attacks of the Thunder Breathing. Through this, Gyutaro also then develops a psychic connection with her and allow him to see, hear and sense everything she does and even possess and take control of her body in order to better coordinate their attacks.
- Decapitation Immunity: This ability allows Gyutaro to detach his "core" from his own body, effectively protecting him from being killed via decapitation due to his main core not being present in his own body. This effectively insures that both he and his sister cannot be killed by decapitation unless both of their heads are cut off simultaneously.
Extrasensory Perception: Gyutaro was noted to possess the natural subconscious ability to process the vast sensory information both his and his sister's immensely honed senses send to him through their psychic connection. Allowing him to perfectly instinctively act on the information given to him on pure instinct and maneuver both his and her attacks and actions perfectly while in battle, as seen during their battle against Tengen, Tanjiro, Inosuke and Zenitsu, where he perfectly coordinated his attacks with Daki's to both launch several simultaneous assaults on all four opponents despite the both of them fighting in separate locations and facing two entirely different pairs of opponents by manipulating his sister's actions and controlling her sashes to strike in tandem with his blood splash slashes to create powerful surprise attack combos. He was also noted to possess immensely honed senses, having been able to maintain full perception of everything happening around him and keep track of every single person he and his sister was fighting through their psychic connection, allowing him to keep track of both battles with little effort and easily detect any sneak attacks and human/Demon presences when they approached either of them.
Flesh Manipulation: Gyutaro, like his sister, possesses the ability to manipulate the flesh in his body at will, but at a more advanced level than his sister. He has used this ability to shape a part of his flesh into two extremely sharp sickles.
Immense Speed & Reflexes: Gyutaro has amazing movement speed, being able to evade attacks from a weakened Tengen Uzui effortlessly in the blink of an eye. He can move so fast that it makes it seem like he teleported.
Immense Regeneration: Like all Demons, Gyutaro also has astounding regenerative powers, with his own recovery speed being further enhanced upon joining the Twelve Kizuki and receiving more of Muzan Kibutsuji's blood that then further strengthened this ability, allowing him to easily fend off and adapt to powerful wisteria flower based poisons and easily regrow lost limbs in a matter of seconds.
Unlimited Stamina & Endurance: Gyutaro possesses absolute and infinite stamina and vitality, never tiring and always remaining in optimal physical and mental condition.
Blood Manipulation: Gyutaro's main Blood Demon Art is to manipulate his own blood. He mainly uses this ability to create heavy flying solidified blood slashes and manipulate them freely, he has shown to create dozens of these during battle which overwhelmed both Tanjiro Kamado and Tengen Uzui. He can also use his own blood to defend himself from incoming attacks like he did against a barrage of Kunai.
Poison Manipulation: Gyutaro possesses the ability to generate extremely lethal poison using his blood and he uses it to coat his sickles and gravely injure his opponents, as shown when he fatally wounded Tengen Uzui, a Hashira with high tolerance to poison, from just a few slashes.
Sickle Proficiency: Gyutaro’s main form of combat revolves around a pair of Japanese sickles, known as Kama. Since Gyutaro has lived for over a hundred years, he has polished his skill in dual wielding his sickle to supernatural levels, being able to easily engage in combat at subsonic speeds and easily overwhelm Tengen Uzui while still effectively utilizing his dual sickles for both offense and defense.
Gyutaro's techniques combines his sickles in conjunction with his blood manipulation ability to poison his opponents. He can also use his blood for offense and defense.
- Flying Blood Sickles (
飛び 血 鎌Tobi Chigamo?) - Gyutaro releases multiple blades of blood from his sickles. These blades can be manipulated freely by him.
- Rampant Arc Rampage (
跋 弧 跳 梁Batsuko Chiyō Ryō?) - Gyutaro slashes downwards rapidly in all directions, creating a dome of blood blades around him that defend from incoming attacks.
- Rotating Circular Slashes: Flying Blood Scythes (
円 斬 旋 廻・ 飛び 血 鎌Enzan Senkai - Tobi Chigamo?) - Gyutaro releases blood blades all over his body simultaneously. This technique has massive range and he is even able to activate it as his head is being cut off.
- Muzan Kibutsuji has stated in Chapter 98 that he was the True Holder of the Position of Upper Rank 6, not his sister.
- Gyutaro was one of Muzan's favorite Upper Rank, since he highly values his circumstance and greedy personality.
- The word 'Gyu' refers to the barkers who would entice customers to enter the entertainment district. After working as a Gyu, he took the word and used it to name himself as Gyutaro.
- (To Tanjiro Kamado, Zenitsu Agatsuma, Inosuke Hashibira and Tengen Uzui) "The sufferings of my past were lessons on what to do to others as revenge! If I don't cause misery equal to my own unhappiness, I'll never balance the scales! That's how it is when you're a Demon! And we kill anyone who tries to tell us otherwise." 
- (To himself, recalling his human life) "It turns out I was good at fighting. So I worked as a debt collector. People found me creepy. They feared me. I liked that feeling. I became proud of my ugliness. And with such a beautiful sister, I began to lose my sense of inferiority. It felt like our lives were finally going in a good direction."