Hotaru Haganezuka, a 37-year-old swordsmith from the Swordsmith Village, hails from the prestigious Haganezuka Clan. His parents suffered a mental breakdown due to his extreme temperament at age two, leading village chief Tecchin Tecchikawahara to raise him. Tecchin bestowed the name "Hotaru," which Haganezuka dislikes for its perceived cuteness. He perpetually wears a spotted head scarf, conceals his identity behind a hyottoko mask, and dons a straw hat decorated with floral wind chimes—a childhood calming mechanism. Rigorous mountain training developed his notably muscular physique. He sustained severe injuries during an attack by Upper Rank Five Gyokko, resulting in deep scars across his body and face and the loss of his left eye's function. Hot-blooded and socially inept, Haganezuka exhibits obsessive dedication to sword craftsmanship. He reacts with violent outbursts when his blades are damaged, especially those wielded by Tanjiro Kamado, whom he frequently chases with knives. These rages subside through eating mitarashi dango or physical intervention like tickling, which causes him to go limp. Despite his volatility, he respects the sacrifices of Demon Slayers and harbors unspoken gratitude toward Tanjiro for trusting his work. His forging focus is absolute, ignoring even life-threatening dangers. Assigned to forge Tanjiro's Nichirin Sword after Final Selection, he expressed intense disappointment when it turned black instead of red. Subsequent breakages—during battles with Lower Rank Five Rui, Upper Rank Six Gyutaro and Daki, and on the Mugen Train—triggered repeated confrontations where he attacked Tanjiro or sent furious letters refusing repairs. In the Swordsmith Village Arc, after initially avoiding Tanjiro, he overheard the latter defending swordsmiths' importance, moving him deeply. He emerged from mountain training to retrieve a 300-year-old sword from Yoriichi Type Zero, pledging to refine it. During Gyokko's attack, he continued sharpening the blade despite catastrophic injuries, including eye loss. The unfinished sword was used by Muichiro Tokito to help Tanjiro decapitate Upper Rank Four Hantengu, inciting Haganezuka's protests. Later, at the Butterfly Mansion, he delivered the fully polished sword to Tanjiro, passive-aggressively detailing its superior steel and historical significance while lamenting damage incurred during premature use. In the Kimetsu Academy spin-off, he serves as a home economics teacher, fiercely protective of kitchen tools and skilled in culinary instruction, though coworkers find him difficult. Trivia includes his surname meaning "steel" referencing his profession, childhood wind-chime use for calming, and an irrational fear of his own razor-sharp kitchen knives.