Genya belonged to a family of seven children with a hardworking mother who never slept and an abusive father who would beat her and his children. His father is hated among the villagers and is 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 is 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 is sliced open and they were cut down immediately. In the ensuing panic, Genya watched helplessly as the 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 Breathing Styles, he discovered that he could eat Demons and gain Demonic abilities. He is discovered by the Stone Hashira, 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 reunited with Sanemi and tried to apologize, but Sanemi rejected him and said he had no brother. After this, Genya decided to become a Hashira as a way 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 Ubuyashiki, 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, 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 is present, the scarred youth ripped one of his teeth out and discarded it, accidentally hitting Tanjiro with it. He turned when his name is 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. Genya also did not show up for any meals, having told the villagers he wouldn't eat at all. When Tanjiro went to bring him some leftovers, he isn'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, exclaiming 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 is 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 Rank'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 is 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. Genya shot at the surprised Demon, blowing his head to bits. He noted that the headless Demon is 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 attacking 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 is by introducing himself, promising to kill Aizetsu.
Genya manages to defeat his opponent, spearing him to a tree and ripping his head off. He turns to face Tanjiro, his appearance now resembling a Demon's, then grabbed him and told him not to push his luck, as he would kill the Upper Rank. He goes on to deny Tanjiro's defeat of Upper Rank 6 and states that he will become a Hashira 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 is telling the truth. They were then interrupted by an electricity attack from Sekido, with Tanjiro heading off to find the Upper Rank's fifth body, telling Genya not to attack Nezuko.
Genya heads 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 his tactic 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 Hashira so his sibling would accept him.
Luckily, he is saved at the last second by Tanjiro and Nezuko, but after Hantengu’s hatred forcefully combines with his fellow aspects into Zohakuten, he attacks Tanjiro. Before Genya can shoot the Demon, Zohakuten uses his Blood Demon Art to turn the trees into dragons. When Tanjiro 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 is shocked that the kind boy had any anger in him.
When Mitsuri 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, Tanjiro, 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. Tanjiro, emulating Zenitsu Agatsuma's Thunder Breathing, 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 wouldn't spread to him. Hantengu, Nezuko, and Tanjiro 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 and even Genya cracks a smile.