Kyojuro Rengoku served as the Flame Hashira in the Demon Slayer Corps, continuing a lineage where his father, Shinjuro Rengoku, previously held the position before retiring after his wife Ruka's death. Ruka instilled in Kyojuro during childhood the principle that the strong must protect the weak, a belief central to his character. Despite his father's later apathy, alcoholism, and verbal abuse directed at him and his younger brother Senjuro, Kyojuro maintained respect for Shinjuro while actively encouraging Senjuro to follow his own dreams. He mastered the Flame Breathing style primarily through self-study using three ancestral manuals after his father ceased training him, achieving the Hashira rank through exceptional dedication. He previously mentored Mitsuri Kanroji, who later became the Love Hashira using a style derived from Flame Breathing. Before the Mugen Train mission, Kyojuro defeated Lower Rank Two demon Hairo and avenged nine slain Demon Slayers by killing a Flute Demon; during this fight, he ruptured his own eardrums to counter the demon's sound attacks, resulting in his characteristically loud voice. Kyojuro possessed a tall, lean-muscular build with bright yellow hair featuring red streaks, black forked eyebrows, and golden eyes fading to red with white pupils. He wore the standard Demon Slayer Corps uniform—a brown gakuran jacket, white belt, and hakama pants—over which he donned a distinctive flame-patterned haori (white to yellow with red ridges at the hem), previously worn by his father. His attire also included red kyahan leg guards with yellow flame designs, pale brown tabi socks, and white zōri sandals with red straps. He transitioned from the standard uniform to a white haori before adopting his signature flame haori upon becoming Hashira. His personality radiated unwavering enthusiasm, charismatic optimism, and a consistently cheerful demeanor, often expressed through an energetic smile. He spoke loudly and decisively, acting with rapid conviction. Beneath this exuberance lay keen logical reasoning and tactical intelligence, enabling swift, precise decisions, especially when lives were at stake. He strictly adhered to the Demon Slayer Corps' conduct, initially advocating for Nezuko Kamado's execution during Tanjiro's trial. However, he demonstrated nuanced judgment, accepting Nezuko as an ally after witnessing her protect humans on the Mugen Train. He readily recognized potential, enthusiastically inviting Tanjiro, Zenitsu, and Inosuke to become his apprentices after their encounter. As a combatant, Kyojuro ranked among the Corps' most formidable swordsmen. His immense speed caused ground tremors and outpaced Upper Rank demons, his superhuman strength allowed him to halt a moving train and manifested in an "unbelievable" grip noted by Akaza despite mortal injuries, and his extraordinary endurance let him fight through ruptured eardrums, gunshot wounds, explosions, and fatal internal damage without faltering. He possessed extrasensory perception for detecting demonic presence, surpassing even Tanjiro's sense of smell. His mastery of Flame Breathing included forms like Unknowing Fire for high-speed decapitation, Rising Scorching Sun as an ascending slash, Blooming Flame Undulation for circular defense, Flame Tiger manifesting fiery tiger-shaped slashes, and Rengoku—a devastating dashing strike carving terrain. His indomitable willpower, noted by Akaza as nearing the "Realm of the Highest," allowed him to persist far beyond physical limits. During the Mugen Train mission, Kyojuro first eliminated a demon terrorizing railway workers and rescued a bento vendor's granddaughter before boarding. He encountered Tanjiro, Nezuko, Zenitsu, and Inosuke aboard the train, enthusiastically discussing breathing techniques and offering to train Tanjiro. When Lower Rank One Enmu used a Blood Demon Art to induce sleep, Kyojuro's dream reflected a desire for mentorship, envisioning Tanjiro and his companions as younger brothers calling him "aniki." After awakening via Nezuko's intervention, he coordinated defense against Enmu's fusion with the train, instructing Tanjiro and Inosuke to locate the demon's neck while protecting passengers. Following Enmu's defeat, he stabilized Tanjiro's abdominal wound using breathing techniques. Upper Rank Three Akaza then confronted Kyojuro, offering demonhood to transcend human limits. Kyojuro vehemently rejected this, asserting the nobility of human mortality and the value of protecting others. Their battle severely injured Kyojuro, costing him an eye and damaging internal organs. Despite this, he fought Akaza to a standstill, nearly decapitating him and pinning him as dawn approached. Akaza tore off his own arms to escape sunlight. Kyojuro's final act prevented Akaza from killing Tanjiro. Mortally wounded with a pierced solar plexus, Kyojuro assured Tanjiro that his survival meant victory, entrusted him with visiting the Rengoku estate for knowledge on Hinokami Kagura, and urged all present to grow stronger and protect lives without surrendering. His last words emphasized caring for his father's health and Senjuro's freedom to pursue dreams before succumbing while recalling his mother's teachings. Kyojuro's death profoundly impacted the Demon Slayer Corps, solidifying his legacy as a role model balancing strict duty with genuine compassion. His unwavering principles, passionate mentorship, and ultimate sacrifice reinforced the importance of protecting the vulnerable, significantly influencing Tanjiro's resolve. The Corps leadership honored his success in safeguarding every passenger and comrade during the Mugen Train incident.