Zenitsu Agatsuma is a young Demon Slayer Corps member defined by profound cowardice and low self-esteem. Orphaned early, he fell into significant debt after being deceived by a woman he loved. Former Thunder Hashira Jigoro Kuwajima rescued him by settling his debts and training him in Thunder Breathing. During training, Zenitsu was struck by lightning while hiding in a tree, transforming his black hair to yellow. He trained alongside the senior disciple Kaigaku, sharing mutual dislike yet aspiring to fight alongside him as an equal. Despite Jigoro's patient teaching, Zenitsu mastered only the first form of Thunder Breathing. His appearance features fair skin, gold-fading-to-brown eyes, and yellow hair with orange tips. He wears a modified Demon Slayer uniform: a pale brown gakuran jacket, hakama pants, and a signature white-triangle-patterned haori in a yellow-to-orange gradient. After exposure to demon poison during the Mount Natagumo battle, his limbs temporarily shrank, later restored to normal by Shinobu Kocho's antidote. As an adult, he develops a plump physique with minimal facial hair. Personality-wise, Zenitsu exhibits chronic fear, frequently predicting his own early death and attempting to flee danger. He voices intense self-loathing over his perceived incompetence and cowardice, often through loud, annoying outbursts. Driven by a fear of dying single, he desperately seeks marriage, developing a strong romantic fixation on Nezuko Kamado. Despite this, he shows fierce loyalty and protectiveness, shielding Nezuko's box from Inosuke despite severe beatings and demanding Daki apologize to an abused human girl. His urban upbringing grants him street smarts, contrasting with Tanjiro and Inosuke. Learning of Jigoro's death via Kaigaku's betrayal triggers a shift to a serious, focused demeanor, culminating in a stern confrontation with his former senior. Zenitsu's combat relies on Thunder Breathing, initially limited to the First Form: Thunderclap and Flash. He refines this into successive variations: Sixfold, Eightfold, and Godspeed—the latter risking leg damage if overused. His true power emerges during unconscious states triggered by extreme fear; suppressed nerves enable instinctive combat leveraging his enhanced hearing. This hearing detects environmental details, emotions, and distinguishes demons from humans. He develops an original Seventh Form: Honoikazuchi no Kami, resembling a lightning dragon, to defeat Kaigaku. By the series' end, his speed surpasses Upper Rank demons and contributes significantly against a weakened Muzan Kibutsuji. His character evolves from fear-induced paralysis to determined resolve. Early victories, like in the Drum House and Mount Natagumo, occur only while unconscious. Key moments include awakening from Enmu's dream manipulation on the Mugen Train to protect others and consciously fighting during the Infinity Castle arc. His battle against Kaigaku showcases his matured resolve; awake, he denounces his senior's disrespect toward Jigoro and defeats him with his self-created Seventh Form. He participates intensely in the Hashira Training arc, advancing to its highest levels. His journey concludes with survival and involvement in Muzan's final defeat. Relationships drive Zenitsu. He views Jigoro as a grandfather figure, internalizing his teachings to honor him. His bond with Tanjiro begins reluctantly but deepens into mutual trust, cemented when Zenitsu protects Nezuko's box. His romantic devotion to Nezuko remains constant, motivating protective actions. Rivalry with Inosuke involves frequent squabbles but evolves into camaraderie during battles. Kaigaku represents betrayal and loss, fueling Zenitsu's final confrontation. He also interacts with Kanao Tsuyuri during rehabilitation at the Butterfly Mansion, respecting her quiet strength.