Hotaru Haganezuka, a swordsmith from the esteemed Haganezuka Clan serving the Demon Slayer Corps, was raised by Swordsmith Village chief Tecchin Tecchikawahara after his parents struggled with his temperament at age two. He developed an early passion for sword-making under village masters, specializing in Nichirin Sword forging and polishing. He intensely dislikes the name "Hotaru" (firefly), given by the chief, for sounding too cute. He typically conceals his face behind a Hyottoko mask and head scarf, wearing a black yukata with a sunflower-patterned haori and a large woven hat adorned with floral wind chimes when traveling. After rigorous mountain training, his physique becomes noticeably more muscular, with a lock of black hair visible. Unmasked, he reveals sharp eyes, thick eyebrows, and long black hair. He sustained severe injuries during Upper Rank demon Gyokko's attack, including deep body scars, an X-shaped facial scar, and permanent loss of his left eye. His personality is intensely focused and volatile, marked by extreme dedication to his craft. He speaks extensively about blades, prioritizing sword-making above all else, including personal safety—this single-mindedness left him unaware of Gyokko's repeated attacks while refining a blade. He reacts with violent rage if his swords are damaged or disrespected, particularly blaming Tanjiro Kamado for multiple broken blades, chasing him with knives. Such outbursts can be halted by eating mitarashi dango or physical intervention like tickling. Socially inept, his temper has cost him clients and left him unmarried, though he secretly values Tanjiro's trust. Despite his asocial behavior, he holds deep respect for Demon Slayers' sacrifices, defending their work as noble when a potential match dismissed swords. Hotaru first appears delivering Tanjiro's Nichirin Sword after Final Selection. Excited by Tanjiro's red hair and eyes—traits linked to rare bright-red blades—he becomes enraged when the sword turns black. He later pursues Tanjiro with knives after the sword breaks during battles against Lower Rank Five Rui and in the Entertainment District. Following the Entertainment District battle, Hotaru refuses via letter to forge another sword after Tanjiro's blade chips, prompting Tanjiro's visit to the Swordsmith Village. There, Hotaru hides upon learning Tanjiro seeks him but is moved overhearing Tanjiro passionately defend swordsmiths' importance. Returning visibly muscular after mountain training to create stronger blades, Hotaru intervenes when Tanjiro discovers a 300-year-old sword inside a training doll, insisting on refining the severely rusted blade. During Gyokko's attack, he continues sharpening despite grave injuries, briefly shielded by fellow swordsmith Kozo Kanamori. Persisting even after being blasted away, his unfinished sword is later used by Tanjiro to behead Upper Rank Four Hantengu—Hotaru protests its premature use, but his efforts prove crucial to the demon's defeat. His relationship with Tanjiro evolves from initial hostility over broken blades into unspoken mutual respect. He shares connections with other swordsmiths like Kozo Kanamori (who often restrains him) and the village chief Tecchin, whom he frequently badmouths due to resentment over his name. His background, skills, and actions during the Swordsmith Village attack underscore his unwavering commitment to supporting Demon Slayers through his craft.