Early on in the series, Genya had a short build and was only slightly taller than Tanjiro Kamado. He had short, messy back hair that what shaved off at the sides, thin eyebrows, and a scar across his face.
Later on, he reappeared with a wilder look; he underwent a growth spurt, making him taller than most, and now has longer hair reaching down to his neck. He also gained a large, muscular build along with this as well as various scars all over his body.
Genya is a foul-mouthed and ill-tempered individual; a "lone wolf" who refuses any help from others and does not show any interest in being polite or having any manners, much like his older brother. During the Final Selection, he roughly handles Kanata Ubuyashiki, grabbing her by her hair and yelling at her even though she is a child.
He becomes more open to cooperation during the battle against Hantengu, an Upper Moon demon, where he works alongside Tanjiro, Mitsuri Kanroji, and Nezuko Kamado. Sanemi's refusal to acknowledge him also makes Genya insecure as he comes to realize just how weak he is compared to his companions. Nonetheless, he does whatever he can to help his allies, even if it gets him horribly maimed in the process.
Genya belonged to a family of nine children, with a hardworking mother who never slept and an abusive father who would beat her and his children. He was hated among the villagers, and was eventually stabbed to death by an unknown assailant. After that, Sanemi and Genya vowed to protect their siblings and mother.
One day, their mother went missing and Sanemi left to find her. As suspicion and worry began to cross the minds of the children, alone and unprotected, a loud knock was heard from their door. Four of his siblings ran to the door despite Genya's protests that it might not be their mother, but before he could react, the door was sliced open and they were cut down immediately. In the ensuing panic, Genya watched helplessly as this figure—which he assumed to be a wolf—attacked them. Before it could attack Genya, however, Sanemi tackled it out of their window to the streets below and ordered Genya to run away.
Seeing his bloodied siblings, subconsciously aware that they were already dead but unable to fully process it, Genya set out for a doctor but soon stumbled across Sanemi, with their mother lying dead at his feet. Genya, in his confusion and panic, labeled Sanemi a murderer and cradled their mother's body in his arms as she dissolved in the rising sunlight.
Later on, Genya realized his mistake and decided to join the Demon Slayers to try and reconcile his relationship with his brother. Although he found he had no talent for Breath Styles, he discovered that he could eat Demons and gain Demonic abilities. He was discovered by the Rock Pillar Gyomei Himejima. Since Genya couldn’t use Breaths, Gyomei didn’t make him his Tsuguko, but he still decided to train him. At some point he met back with Sanemi and tried to apologize, but Sanemi rejected him calling him wearing and saying he had no brother. On this, Genya decided to become a Pillar to regain some connection with his brother.
Genya participated in the Final Selection exam, managing to survive the seven-day trial. He impatiently inquired about receiving his Nichirin Blade, angrily hitting both his Kasugaigarasu and Kanata when his demand went unfulfilled. Tanjiro intervened, threatening to break his arm if he didn't immediately let go of Kanata. When Genya instead dares him to, Tanjiro obliges, leaving the irate youth to nurse his broken limb. Kiriya, undeterred by the rough treatment his sister received, simply continues with his instructions.
Later, at the Butterfly Estate under unknown circumstances, Genya intentionally bumped into Tanjiro, ignoring his greeting afterwards. While training with Gyomei Himejima, he later received the report of Kyojuro Rengoku's death via kasugaigarasu.
Genya later appeared at the Swordsmith Village, ignoring Mitsuri when she asked for his name—though out of shyness rather than spite—and is found by Tanjiro in the hot springs. Before realizing Tanjiro was present, the scarred youth ripped one of his teeth out and discarded it, accidentally hitting Tanjiro with it. He turned when his name was called, angrily told his fellow Demon Slayer to drop dead, and plunged his head into the hot water when he persisted in talking with him. He also didn't show up for any meals, having told the villagers he wouldn't eat at all. When Tanjiro went to bring him some leftovers, he wasn't in his room.
A few days later, Genya listened with impatient anger as Tanjiro explained the situation with Hotaru Haganezuka and the rusted Nichirin Blade, and then exclaimed that he didn't care and that Tanjiro shouldn't talk to him as if he were his friend. He reminded his companion that he had broken his arm, told him not to call him by his first name, broke the rice cracker Tanjiro offered him, and told him to get out. He fell silent when Tanjiro pointed out his regrown tooth, telling him that he must be seeing things, but was disgusted when his companion showed him the tooth in question. Telling him to throw it out, the scarred youth kicked Tanjiro out of his room.
Genya appeared during Tanjiro and Nezuko's battle against Hantengu, aiming his gun at the Upper Moon's staff-wielding form, Sekido. He shot a cluster of bullets, which completely decapitated Sekido but failed to completely do so with Karaku. Moving in to finish the job with his Nichirin Blade, Genya ignored Tanjiro's advice, proceeding to complete Karaku's decapitation. The two Demons became four, and Genya was stabbed through the chest by the spear-wielding Hantengu, Aizetsu.
Aizetsu attempted to pull the spear out of Genya to attack Nezuko, only to be unable to do so. The scarred youth shot at the surprised Demon, blowing his head to bits. He noted that the headless Demon was starting to pull his spear free, getting a horrendous slash wound across his chest as a result. Genya began to pray, avoided another attack from his opponent, and attacked the Aizetsu from behind. However, Sekido intervened using his electricity and Aizetsu attacked him once more, this time managing to smash him into a wall. Undeterred, Genya began to pray again, answering his opponent's question of what he was by introducing himself, promising to kill Aizetsu.
Genya managed to defeat his opponent, spearing him to a tree and ripping his head off. He turned to face Tanjirou, his appearance now resembling a Demon's, then grabbed him and told him not to push his luck, as he would kill the Upper Moon. He goes on to deny Tanjiro's defeat of Upper Moon Six and states that he will become a Pillar before him. When Tanjiro happily agrees to this, even offering his support, Genya tries to call his bluff, only to see in his comrade's eyes that he was telling the truth. They were then interrupted by an electricity attack from Sekido, with Tanjiro heading off to find the Upper Moon's fifth body, telling Genya not to attack Nezuko.
Genya headed northeast at Tanjiro's instruction to find the fifth Hantengu, noting with frustration that he still couldn't see him, only to finally notice him: A small version of Hantengu's original form, only about the size of a rat. He shot at his tiny foe, then attempted to cut his head off with his blade, only for it to break. After firing another shot at the tiny Hantengu, he discovers it would not work. Sekido ambushes him from behind, and Genya only has time to apologize to Sanemi in his mind for "what happened back then", noting that he only wanted to become a Pillar so his sibling would accept him.
Luckily, he was saved at the last second by Tanjiro and Nezuko, but after Hanetengu’s hatred forcefully combines with his fellow aspects into Zohakuten, he attacks Tanjiro. Before Genya can shoot the demon, Zohakuten used his Blood Demon Arts to turn the trees into dragons. When Tanjirou proclaims his hatred for the demon and his mentality—that demons were protecting themselves from humans that hunted them down, even though they regularly attacked and consumed humans—Genya was shocked that the kind boy had any anger in him.
When Mitsuri Kanroji arrives and starts fighting Zohakuten, he goes to protect her when the demon stuns and nearly kills her. Mitsuri decides to hold off Zohakuten while Genya, Tanjirou, and Nezuko pursue Hantengu's main body. When they get constricted by the wood dragons, Genya simply eats the dragon to free themselves and absorb its demonic abilities. After they finally destroy the wood dragon and get to Hatengu's protective orb, they find that the wooden sphere is empty. When they spy the small demon, Genya throws a tree at the demon in his anger. He misses, however, so the three give chase once again. Tanjirou, emulating Zenitsu’s Breath of Thunder, rushes at the demon with blinding speed.
However, Hantegu's resentment causes him to split again, creating a larger body around himself, while he is hidden inside its heart. When Hantengu grabs Tanjiro, Genya goes to his aid. He pries the demon's hands off and then rips off his arms, while Nezuko shoots fire at him. Due to becoming a partial demon as a result of consuming them, the flames start hurting him, forcing him to retreat so the fire won’t spread to him. Hantengu, Nezuko, and Tanjirou fall off a cliff. Genya, heavily injured, is unable to help further in the fight. After Tanjiro finally slays the demon and Nezuko reveals her resistance to the sun, everyone celebrates—even Genya cracks a smile.
Sanemi is Genya's older brother and only living sibling. After a demon attack befell their family, Sanemi and Genya were the only ones who survived the incident but after Genya branded his sibling a "murderer", their relationship began to worsen and decline until the two split and went their own ways. It was revealed during the battle with Kokushibou that despite the whole incident, Sanemi still cares for his younger brother and actually wants him to live a normal life. Genya was moved to tears and apologizing after hearing Sanemi reasoning behind his rudeness.
At first, Genya treated Tanjirou like an annoying pest and wouldn't even talk to him. This didn't discourage Tanjiro who kept wanting to connect to Genya, which just pissed him off. However, throughout the fight with the Demons, Genya starts connecting to Tanjiro and gains respect for him. Tanjiro also tried to mend Genya's relationship with Sanemi, although his actions earn him a mouthful from the higher-ups.
Abilities and Powers
Demonic Transformation: By eating a demon, Genya can temporarily gain demonic attributes from it. Depending on the strength of the individual, he can gain regeneration and physical strength equal to one. In this form, Genya was capable of overpowering Aizetsu briefly.
- Upper Moon Four Empowerment- Later after consuming the wood created by Zouhakuten Genya had enough strength and speed to throw 3 trees in quick succession,and in his battle against Kokushibo, The full extent of his demonification upone eating some of the Upper Moons flesh was shown when despite Upper Moon one viciously bifurcating him across his abdomen and amputated both his arms, he still did not die, showing that like a genuine demon,Genya could now only be killed by decapitation.
- Upper Moon One Empowerment- Upon eating the hairs of Upper Moon One,Kokushibo Genya's demon abilities were even further enhanced, as seen when he managed to reattach his severed body in mere seconds and was even able to intercept the telepathic communication of Muzan Kibutsuji, and upon eating a piece of the Upper Moons Flesh Sword,Genya even began taking up several of Upper Moon One's physical characteristics such as his eye's and hair and gaining his ability to detach his cells and merged them to objects.
- Flesh Gun- Upon eating more of Upper Moon Ones cells, Genya displayed the demons ability to detach his cells and merge it with his gun,with the gun having the same characteristics of Kokushibo's sword of having multiple eyes on top of the barrel and muzzle,which presumably increases its accuracy and power.
- Demon Slayer Mark - Upon eating more of Kokushibo's cells, Genya also gained access to his demon slayer mark, that appears as a flame like tattoo design on top of the right side of his forehead which presumably also increases all of his physical and mental abilities as well.
Blood Demon Arts
Due to having eaten the flesh and cells of at least Two Upper Moons (Kokushibo & Hantengu ) Genya has seemingly developed his own Blood Demon Art while in the heat of Battle against Upper Moon One and similar to Nezuko's own Blood Demon Arts,Genya's own are specifically tailored to battle and do damage to other Demons.
- Wooden Flesh Bullets - Due to Eating the cells of Upper Moon One & Four, Genya has developed a unique technique where he merges his demon transformed cells with his sun steel bullets that when fired, its direction and velocity can be freely manipulated by Genya to easily move around obstructions between the bullet and the intended target,as seen when his bullets moved around Kokushibo's sword to directly bury itself into his flesh,once inside its target the bullets can immediately grow into a gigantic tree similar to the tree creatures of Upper Moon Four's Zou Hakuten form to then paralyze its target.
- Blood Roots - Genya can further alter his embedded Flesh Bullets by having them take root inside his target and suck the demons blood from inside their bodies,effectively preventing them from activating and accessing their own Blood Demon Arts.
- Genya is one of the few characters in the series who does not primarily use a sword and is the only character so far who cannot use any of the Breaths.
- It seems that Genya is also shy around women, breaking out into a blush and be unable to speak with them. This is seen when interacting with the Love Pillar, Kanroji Mitsuri and the younger girls who work at the Butterfly Estate.