Kyojuro was the eldest son of Shinjuro Rengoku. His father was known as the Flame Hashira of the Demon Slayer Corps until his unexpected retirement. Kyojuro also had a younger brother who trained alongside him until their father ceased to teach them.
Shinobu Kocho notes that Tanjiro is on trial, which Kyojuro sees no need for as he was protecting a Demon, which is a clear violation of conduct; he states that it was a matter they could deal with themselves. He then agrees with Gyomei Himejima's idea to kill Nezuko Kamado, witnessing shortly afterward Tanjiro's coughing fit, and remains silent when Mitsuri Kanroji asks if they should be dealing with their subordinate's trial without Oyakata-sama's input. After Tanjiro headbutts Sanemi Shinazugawa, Kyojuro looks at Mitsuri when she laughs with a neutral expression.
Kyojuro is sent to ride the infinity train after a number of Demon Slayers who were sent there to investigate it fell silent.
As he sits on the train, waiting for it to take off and devouring his eleventh bento box while shouting "tasty," Tanjiro, who'd heard that he was going to be there and consequently followed him along with Inosuke Hashibira, Zenitsu Agatsuma and Nezuko in order to ask him about Dance of the Fire God, comes up behind Kyojuro. After the Hashira finishes his food and Tanjiro explains his question, Kyojuro energetically answers that he doesn't know what Dance of the Fire God is and that this is his first time hearing about it. He says that he's delighted that Tanjiro applied his father's dance to his fighting style, then swiftly changes the subject and tells Tanjiro that he should become his Tsuguko. Tanjiro tells him to slow down and asks if he can at least try and help a little more to answer his question.
Kyojuro says that Breath of Flames has a long history, and says that there has always been a Hashira in every age who uses Breath of Water and one who uses Flames. He explains that the other Breath Styles were derived from either those two or Breath of Wind, Stone or Thunder. Kyojuro then asks Tanjiro what color his Nichirin Blade is, to which the latter responds that it's black, and Kyojuro laughs out and says that that must be unfortunate. Tanjiro asks why that is, and, as the train starts to take off, Kyojuro explains that he's simply never seen a swordsman with a black blade become a Hashira before, and that he's heard people whose blades turn black don't even know which Breath Style to master. Seeing Tanjiro's face, he tells him not to worry and that he'll train him up back at his house.
Inosuke sticks his head out of the train's window, and Zenitsu tells him to get back inside because it's dangerous for him, and Kyojuro agrees, since he doesn't know when the Demon is going to appear. Zenitsu is shocked by this, and Kyojuro explains the situation and the disappearances of the Demon Slayers sent to investigate the train. Zenitsu starts screaming that he wants to get off the train.
The conductor then comes over to the four of them to stamp their tickets. Tanjiro smells something bad, and Kyojuro appears to sense it too as he asks the conductor to step aside as he draws his sword. A large Demon appears at the other end of the carriage, and Kyojuro asks it if it was hiding its Blood Demon Arts as he found it hard to notice its presence, before darting forward to behead it with his Breath of Flames: Unknowing Fire. The Demon's body falls to the floor, and Tanjiro, Inosuke and Zenitsu all crowd Kyojuro, praising his swordsmanship and pestering him to let them become his disciples. The Hashira enthusiastically agrees and says that he'll look after all of them.Sometime later, when the four are all asleep, Kyojuro dreams that he's back at his house, telling his father, Shinjuro, about his promotion to Hashira. Shinjuro dismisses him, saying it's pointless and doesn't matter because neither Kyojuro or himself will ever become great people anyway. Kyojuro gets up and walks out of the house, out into the garden, his little brother, Senjuro, coming to greet him and asking if their father was happy about the news and wondering if he would acknowledge him too if he also becomes a Hashira. Kyojuro remembers how his father used to be, a happy man who was passionate about training his children as swordsmen, and how he changed after their mother's death. He admits to his brother that Shinjuro wasn't happy to hear the news, but enthusiastically adds that Kyojuro's passion won't die because of that and that he'll never be disheartened. Senjuro still looks upset, but his big brother comforts him, saying Senjuro's not like him because he has an older brother to look out for him. He reassures him that whatever path he takes, he'll be a wonderful person, and, as Kyojuro hugs his brother, he tells him to do his best and keep on living, no matter what.
Still within the dream, Kyojuro begins to tutor Senjuro in his sword skills. In reality, a young girl, who's working for Enmu, ties her wrist to his unconscious body with a special piece of rope that was made by the sleep Demon, allowing her to enter Kyojuro's dream. Enmu has sent her in to destroy Kyojuro's spiritual core, which, when destroyed, would render him a lifeless shell unable to put up a fight against the Demon. The spiritual core lies in the area of unconsciousness just outside of Kyojuro's dream realm, and she quickly slips past him and Senjuro without him noticing. She finds his unconsciousness, which lies past the wall surrounding his garden, and breaks into it, finding herself in an unusually hit and flame-covered unconscious area.
The girl locates his spiritual core, but, as she goes to break it, in reality, Kyojuro grabs her by the neck and lifts her off the floor, causing the version of her inside his dream to fall still because she is suffering stress in the real world. Kyojuro does this on instinct because he would be unable to fight if she destroyed his spiritual core, and is therefore unable to cause her any more harm because he has no reason to kill another human. Kyojuro and the girl are stuck in a deadlock, both unable to move.
Tanjiro soon wakes up from the dream he was having and asks Nezuko to burn the rope connecting Kyojuro and the girl, removing the girl from the Hashira's dream. After the young Demon Slayer's confrontation with Enmu, Kyojuro too wakes up and tells Tanjiro that he, Zenitsu and Nezuko will defend the eight train cars from the Demon while Tanjiro and Inosuke go down to the front of the train to defeat him.
After Tanjiro and Inosuke have successfully defeated Enmu and escaped the out of control Demon train with Nezuko, Zenitsu and Kyojuro, the Hashira checks on an injured Tanjiro as Inosuke goes to rescue another passenger. He applauds the young Demon Slayer for his recently-acquired ability to use full focus breaths all the time, and says that this is the first step to him becoming a Hashira.
Kyojuro then notices that Tanjiro is bleeding from the abdomen and tells him to focus his breathing in order to heal himself, which the young Demon Slayer manages with some guidance from Kyojuro. The Flame Hashira tells Tanjiro that if he can master his breathing, he'll be able to accomplish many more things and become stronger tomorrow than he was yesterday.
Just as Kyojuro starts to tell Tanjiro that he doesn't need to push himself any further because everyone's been saved, something jumps down just behind the two Demon Slayers with a great force, scattering the dust around them.
The dust clears to reveal Upper Moon Three, who immediately launches himself towards Tanjiro with the intention of punching his head in, but is stopped by Kyojuro's Breath of Flames: Ascending Blazing Sky attack, causing him to leap backwards, away from the Demon Slayers.
The Demon immediately regenerates his injuries and compliments Kyojuro's katana. The Hashira asks him why he was targeting an injured person, to which the Upper Moon says that he thought Tanjiro would get in the way. Kyojuro asks if they have any business together and that he already hates the Demon, despite it being their first time meeting, and the Demon says that he hates weak humans too. The Upper Moon then asks Kyojuro if he wants to be a Demon too.
Kyojuro swiftly declines, but the Upper Moon persists. Kyojuro says that he is the Flame Hashira, Kyojuro Rengoku, and the Demon introduces himself as Akaza. Akaza keeps persisting, saying that Kyojuro would have more time to get stronger as a Demon than a human since he wouldn't have to worry about death.
Kyojuro says that dying is the very beauty of being human, and that he thinks it is the reason that humans are so unique and precious. He says that Akaza shouldn't insult Tanjiro and that he is not weak, and then says that he will never become a Demon.As he begins to use Blood Demon Art: Technique Deployment, Akaza says that he will just kill Kyojuro if he won't become a Demon. He attacks Kyojuro as the Hashira runs towards him with his Breath of Flames: Unknowing Fire attack readied. Akaza jumps back, and says that none of the Hashira he's killed have ever agreed to his invitation to become a Demon and that he finds that hard to understand. He tells Kyojuro to die now, while he's young and strong.
Akaza unleashes Blood Demon Art: Void Style, which Kyojuro counters with Breath of Flames: Blooming Flame Undulation. The Hashira notes that Akaza is attacking him from a distance, punching into the air and moving at a fraction of a blink, and realizes that he'll have to get closer to him if he's to defeat him. Kyojuro suddenly appears in front of Akaza, who compliments his reaction speed as the Hashira attacks him with his flames. The Upper Moon praises his sword skills, and wonders to Kyojuro how sad it'll be if they're lost early, to which the Hashira replies that it won't be sad because it's only natural for people to die.
Tanjiro begins to get up, wanting to help the Hashira out, but Kyojuro tells him to stay where he is, because his wound could be fatal if reopened. Akaza tells him not to take notice of "the weaklings" and to focus on who he's fighting and Kyojuro, angered, attacks the Demon with Breath of Flames: Flame Tiger as Akaza unleashes his Blood Demon Art: War Style.
The residue stirred by their attacks clears, and Akaza tells Kyojuro, who's now bloodied and beaten, not to die on him just yet. Tanjiro and Inosuke panic, the latter wanting to jump in to help Kyojuro but soon realizing that he'd only be a hindrance, as the Hashira and Upper Moon seem to be in an entirely different dimension to him. Akaza says that Kyojuro fighting with his body is pointless since he can't heal his wounds instantly like a Demon, and that humans will never be able to defeat Demons.
Tanjiro tries to pick himself up to go and help Kyojuro, but is unable to generate any strength into his limbs because of the aftermath of Dance of the Fire God, which he used to defeat Enmu. Kyojuro turns back to face Akaza, smiling and saying that he'll fulfil his duties before beginning to ready another attack. Impressed, Akaza compliments his fighting spirit and technique, and laughs as he says that Kyojuro really should've chosen to become a Demon, asking the Hashira to fight with him for all eternity.The two clash again, Breath of Flames: Rengoku meeting Blood Demon Art: Obliteration Style, and the dust this time disperses to reveal that Akaza has stuck his arm through Kyojuro's stomach. Akaza screams that he's doing to die, and asks Kyojuro one more time to become a Demon.
This causes Kyojuro to remember his mother, Ruka, and a flashback shows one of their final moments together. Ruka asks a younger Kyojuro if he knows why he was born stronger than other people, and asks him to think hard about this question. Kyojuro ponders this for a while, then barks out that he doesn't know the answer. Ruka explains that he was given such strength to protect the weak, and that power should be used for the world and for the people, not for abusing it in order to hurt others. Ruka hugs Kyojuro to her chest and, as she begins to cry, tells him that, even though she won't be alive much longer, it was a blessing for her to have been the mother of such a strong, gentle child.Now back in the present, as he summons the last of his strength to swing his sword into Akaza's neck, Kyojuro thanks Ruka as well for being his mother. Kyojuro lets out a shout as he drives his sword deeper in, and, as Akaza tries to hit him with a punch to the face, the Hashira grabs his wrist and stops the attack before it reaches him. Akaza is incredibly impressed by Kyojuro's strength, and then he looks behind Kyojuro and begins to panic as he realizes that the sun is about to come up. Akaza tries to free himself from Kyojuro's hold, but the Hashira holds on tightly to his wrist, not letting him escape.
Tanjiro finally finds the strength to pick up his sword, and starts to run over towards the pair with the intention of dealing the finishing blow to Akaza. With a loud shout of effort, Akaza tries again to pull himself away from Kyojuro, but he fails again as Kyojuro's blade moves deeper into Akaza's neck. Tanjiro yells to Inosuke to move for Kyojuro's sake, and, just as the boar-headed Demon Slayer readies his Breath of the Beast: Pierce and Extract, Akaza manages to pull himself away from Kyojuro.
The Upper Moon quickly jumps backwards, both of his arms now severed, and runs into the forest beside where they'd been fighting, leaving Kyojuro to fall onto his knees, defeated.
As the Demon retreats, Tanjiro throws his sword at Akaza, screaming at him not to run away from the fight and calling him a coward. The young Demon Slayer yells that Kyojuro is better than Akaza will ever be because he fought until the end and didn't let anyone die, and that Kyojuro was the rightful winner of their fight.
Tanjiro lets out a final scream and collapses to his knees, crying, and Inosuke starts to shake. Kyojuro looks over at him and smiles, telling Tanjiro to come over to him so they can have a final chat before he dies.
Kyojuro tells Tanjiro that he should try to visit Rengoku House if he wants to learn more about Dance of the Fire God, since he remembered that his father had some notes that had been left behind by the Flame Hashira who came before he and his son. Tanjiro tells Kyojuro that that's enough, and asks him if there's any way for him to recover from the injuries Akaza inflicted on him, to which he says there isn't and asks Tanjiro to hear him out while he can still talk. Kyojuro asks him to tell his little brother Senjuro that he should proceed down the path his heart tells him to, the one he thinks is right, and to tell his father to take care of himself. He then looks Tanjiro in the eye and tells him that he believes in Nezuko, and fully accepts her as a member of the Demon Slayer Corps after seeing her protect so many humans on the Demon train.
Kyojuro tells Tanjiro to live with his head held high and that he shouldn't worry about him dying here, since it's only natural for a Hashira to give his life protecting his junior. He looks to Tanjiro, then to Inosuke and to Zenitsu, and says that he believes in them and that he thinks they will become the next generation of Hashira of the Demon Slayer Corps.
The Flame Hashira then sees the spirit of his mother in front of him, and asks her if he did well, to which she replies that he did a wonderful job. As the last seconds of his life slip away, Kyojuro smiles.