Kyojuro Rengoku held the position of Flame Hashira in the Demon Slayer Corps. His striking appearance featured bright yellow hair with red streaks resembling flames, golden eyes fading to red, and a flame-patterned haori worn over his standard uniform. Descended from a lineage of Flame Hashira, his father Shinjuro Rengoku previously held the role. His mother, Ruka Rengoku, instilled in him the principle that the strong must protect the weak before her death from illness. After her passing, Shinjuro became neglectful and abusive, abandoning Kyojuro's training. Kyojuro mastered Flame Breathing independently using three-volume family instruction texts, ultimately achieving the Hashira rank. He maintained a close bond with his younger brother Senjuro Rengoku, consistently encouraging him despite Senjuro's perceived lack of swordsmanship talent. He also trained Mitsuri Kanroji, who later developed Love Breathing and became the Love Hashira.

Kyojuro exuded unwavering enthusiasm for his duties, characterized by a cheerful and eccentric demeanor. He adhered strictly to a moral code emphasizing justice, protection of the innocent, and the responsibility of the strong. Though he initially advocated for Nezuko Kamado's execution due to her demon nature, he later accepted her into the Demon Slayer Corps after witnessing her protect humans, demonstrating his capacity to set aside prejudice. Despite his father's apathy and verbal abuse—including Shinjuro dismissing Kyojuro's Hashira achievement as meaningless—Kyojuro consistently showed respect and concern for him. He possessed sharp tactical intellect, enabling swift decision-making in battle and precise threat assessment.

As a combatant, Kyojuro wielded Flame Breathing, a swordsmanship style with nine documented forms. Key techniques included Unknowing Fire (a high-speed decapitating charge), Rising Scorching Sun (an ascending slash), Blooming Flame Undulation (a circular defensive motion), Flame Tiger (slashes forming a tiger-like shape), and Rengoku (a devastating dashing slash that carved the ground). His physical capabilities were exceptional: he generated tremors by running, lifted train carriages, and moved faster than visible perception. He endured severe injuries, such as ruptured eardrums and gunshot wounds, without faltering. During his confrontation with Upper Rank Three Akaza, he fought relentlessly despite losing an eye, sustaining shattered ribs, and suffering a pierced solar plexus. His grip strength immobilized Akaza momentarily, forcing the demon to sever its own arms to escape sunlight. Kyojuro also possessed extrasensory perception, detecting demonic presences faster than Tanjiro Kamado's heightened sense of smell.

On the Mugen Train mission, Kyojuro investigated disappearances linked to Lower Rank One Enmu. Before boarding, he saved a girl named Fuku and her grandmother from a demon, echoing his father's past heroism. During the journey, he allied with Tanjiro Kamado, Zenitsu Agatsuma, and Inosuke Hashibira, swiftly eliminating demons threatening passengers. When Enmu fused with the train, Kyojuro defended multiple carriages simultaneously. After Enmu's defeat, Akaza emerged at dawn. Kyojuro engaged him to protect the incapacitated slayers and civilians, rejecting Akaza's offer to become a demon. He argued that human mortality gives life value—a direct contrast to Akaza's pursuit of strength. Despite fatal wounds, Kyojuro pinned Akaza until sunrise, compelling the demon's retreat. In his final moments, he reassured Tanjiro that his efforts were not futile, urged Senjuro to follow his dreams, and asked Shinjuro to care for his health. His last words encouraged Tanjiro to "set your heart ablaze" and persevere through adversity.