Zenitsu Agatsuma joined the Demon Slayer Corps driven by the need to repay a substantial debt incurred after a woman he admired deceived and abandoned him. This debt was cleared by Jigoro Kuwajima, a retired Thunder Hashira who adopted Zenitsu as his student, training him in Thunder Breathing. A lightning strike during this rigorous training permanently transformed Zenitsu's hair from black to its iconic yellow. He trained alongside Kaigaku, another pupil whose deep resentment fueled constant criticism of Zenitsu's perceived cowardice and informal manner toward their master, whom Zenitsu affectionately called "Gramps," establishing them as narrative foils. Zenitsu displayed extreme cowardice, nihilism, and chronic anxiety, frequently voicing his conviction that demon slaying would lead to an early death. He often tried to flee danger, expressing a strong desire for a quiet, ordinary life. Yet, he held profound respect for Jigoro and his comrades, motivated by a desire to meet their expectations and a deep-seated self-loathing over his perceived failings. His personality included a persistent habit of making unwelcome marriage proposals to women he found attractive, stemming from a fear of dying alone. This trait provided comedic relief but sometimes overrode his timidity, particularly when he sensed romantic rivals. A stark exception to his fear emerged in his fierce protectiveness of Nezuko Kamado; he instinctively defended her box against threats like Inosuke Hashibira and later confronted the Upper Rank demon Daki to shield an innocent girl, demanding Daki apologize for harming her. Initially, Zenitsu's combat abilities were severely limited by anxiety, manifesting fully only during unconscious states triggered by extreme fear. In this state, his heightened sense of hearing granted precise environmental awareness and flawless combat execution, enabling him to effortlessly decapitate demons like the Tongue Demon and the Son of the Spider Family. His sole mastered technique, Thunderclap and Flash, leveraged explosive leg strength for blinding speed. He later refined this into multi-directional variations and Godspeed, increasing its potency. Zenitsu demonstrated remarkable endurance, sustaining prolonged battles such as defending train cars against Enmu and fighting Daki despite severe wounds and broken bones. A pivotal shift occurred upon learning of Jigoro's suicide—prompted by Kaigaku's demonic transformation and betrayal—via a letter delivered by his Kasugai Sparrow, Chuntaro. This tragedy catalyzed Zenitsu's emotional maturation, allowing him to fight demons while fully conscious. Confronting Kaigaku, now Upper Rank Six, in the Infinity Castle, Zenitsu channeled his fury over Kaigaku's disrespect toward Jigoro. In this decisive battle, he unveiled a self-created seventh Thunder Breathing form, Honoikazuchi no Kami (Flame Lightning God), specifically designed to counter Kaigaku's abilities and ultimately decapitate him. This act of vengeance redeemed his master's legacy and showcased Zenitsu's tactical intellect in analyzing and dismantling his opponent's techniques. His endurance and speed proved vital during the final battle against Muzan Kibutsuji, where he persisted despite severe injuries and the demon king's debilitating poison. Following Muzan's defeat and the Demon Slayer Corps' disbandment, Zenitsu's story concluded with his marriage to Nezuko. A modern-day epilogue depicted their descendants living peacefully, free from demonic threats.