Hotaru Haganezuka is a swordsmith of the esteemed Haganezuka Clan from the Swordsmith Village, crafting Nichirin Swords for the Demon Slayer Corps. Born into a lineage of swordsmiths, his intense personality caused his parents distress, leading them to entrust his care to the village chief, Tecchin Tecchikawahara, when he was two years old. Tecchin named him "Hotaru" (firefly), a name Hotaru dislikes for its perceived cuteness. He developed exceptional swordsmithing skills under Tecchin but remained socially inept, causing many Demon Slayers to avoid working with him and leaving him unmarried. He typically conceals his face with a Hyottoko mask, wearing a spotted head scarf, black yukata, and sunflower-patterned haori. When traveling, he dons a large woven hat adorned with floral wind chimes, a childhood calming aid. During the Swordsmith Village Arc, his physique becomes notably muscular after intensive mountain training. Unmasked, he has long black hair, sharp eyes, and thick eyebrows. After an attack by Upper Rank Five demon Gyokko, he sustains severe scars across his body, including an X-shaped facial scar, and loses vision in one eye. His personality centers on an obsessive dedication to sword craftsmanship. He exhibits extreme rage when swords he forges are damaged or broken, often chasing the wielder—particularly Tanjiro Kamado—with knives while blaming them. This fury is typically mitigated by eating mitarashi dango or physical intervention like tickling his sides. Despite his volatile demeanor, he secretly values Tanjiro's continued trust. His forging focus is unparalleled; he ignores external dangers, including demon attacks, and insists on restarting the entire process if interrupted. As a perfectionist, he is deeply pained by irreparable blade imperfections. Beneath his abrasive exterior, he respects Demon Slayers' sacrifices and expresses pride in supporting their mission. He first appears delivering Tanjiro's Nichirin Sword after Final Selection. Excited by Tanjiro's red hair and eyes—traits associated with rare blade colors—he becomes enraged when the sword turns black. He repeatedly pursues Tanjiro with knives upon learning of subsequent breakages during battles against Lower Rank Five Rui and Upper Rank Six Daki and Gyutaro, even sending hostile letters refusing to forge replacements. In the Swordsmith Village Arc, he initially avoids Tanjiro but overhears the latter passionately defending swordsmiths' importance, moving him deeply. Later, he emerges with a transformed physique after training to forge stronger blades. He retrieves a 300-year-old sword from a training doll and refines it using a perilous family technique. During Gyokko's attack, he continues polishing despite severe injuries, including the loss of an eye, shielded by fellow smith Kozo Kanamori. His unfinished sword proves crucial in decapitating Upper Rank Four Hantengu, though Hotaru protests its premature use. In the spin-off *Kimetsu Academy*, he serves as a home economics teacher, fiercely protective of kitchen tools and chasing colleagues who misuse them. He teaches cooking skills and manages a disorganized desk filled with sweets and notes, remaining intimidating yet submissive to the principal. He wields exceptional physical strength and stamina, demonstrated by prolonged pursuits of Tanjiro and overpowering him post-training. As a master swordsmith, his expertise includes forging, repairing, and refining blades, exemplified by his work on the ancient sword under duress. He is skilled in Tantōjutsu, using handmade knives in combat.