Hotaru wears a sunflower-patterned haori and a large, woven hat with hanging floral wind chimes. His head is wrapped in a spotted cloth and his face is concealed by a Hyottoko mask (wooden festival mask), similar to Sakonji Urokodaki.
Unmasked, Hotaru is revealed to have long, black hair coupled with sharp eyes and distinctly-shaped thick eyebrows. However upon being attacked by Upper Rank 5, Gyokko, Hotaru now possesses several deep scars and cuts from the Demon all over his body, including an X-shaped one on his face, and has now lost the use of his right eye.
Hotaru is a rather hot-blooded individual. He seems to show very little care, if any, about anything besides the swords he creates. His passion for swords is so strong that he would be able to speak about them for long periods of time, a trait unique even among the people from the Swordsmith Village.
The same passion that gives him the enthusiasm and extreme dedication in creating swords is also the reason why he is easily enraged (usually in a comical fashion) towards swordsmen that break his creations, especially Tanjiro Kamado, no matter the circumstance.
When angry, he usually calms himself down by eating mitarashi dango or by having others pin him down (one method being tickling his sides). It is later on shown that his skills and concentration on swords is so immense that he would lose clients due to his eccentric and asocial behavior and is actually happy that Tanjiro still trusts him in making his swords. His concentration also prevents him from paying any attention to his surroundings when forging a decent blade, even when a Demon was attacking him.
Hotaru Haganezuka forges Nichirin swords for the Demon Slayers.
Hotaru first appears at Urokodaki's house to deliver Tanjiro's Nichirin Blade after the latter passes the Final Selection. Despite Tanjiro's frequent requests for him to enter the house, Hotaru ignores him and sits outside the door, describing the blade he forged.
After suddenly looking up at Tanjiro, he comments on his red hair and eyes, stating it was good fortune. Excited, Hotaru enters the house while remarking how Nichirin swords would change color and that his sword could turn a bright red, however, after seeing the blade turn jet black, Hotaru shows evident disappointment and gets angry at Tanjiro.
After Tanjiro heals at the Butterfly Mansion, he and Inosuke Hashibira greet their swordsmiths but are met with an angry Hotaru charging at Tanjiro with a knife, furious about his broken blade. After chasing him and then calming down, Hotaru and Kozo Kanamori give the swordsmen their blades.
Hotaru appears when Tanjiro comes back from the Rengoku house, enraged over the loss of another sword and chases Tanjiro again.
Haganezuka is featured prominently in this arc, even having his true face revealed during the attack on the village by two Upper Rank Demons. He proved instrumental in defeating Hantengu, by successfully finishing Tanjiro's blade just in time for the young Demon Hunter to slay him. He suffered severe injuries to his face during the battle, even losing the use of one eye.
Being a swordsmith, Hotaru has shown to have no indication of any special abilities similar to the members of the Demon Slayer Corps. Though after Tanjiro broke his Nichirin Blade multiple times against members of the Upper and Lower Ranks, he underwent special training mountains so he could forge a stronger blade for Tanjiro (albeit he wouldn't admit it).
- Enhanced Strength: After training in the mountains, his increased strength apparently exceeded that of even Tanjiro.
- Master Swordsmith: Hotaru is a skilled swordsmith, able to forge the Nichirin Blade and repaired it multiple times each time it broke against the Lower and Upper Ranks. After Tanjiro chipped his sword in his battle against Upper Rank Six, Gyutaro and Daki, Hotaru trained more to make a stronger sword. Hotaru demonstrated his professionalism and expertise when he polished and sharpened the 300-year-old blade left in Yoriichi Type Zero while completely ignoring the wounds he received from Gyokko.
- Tantōjutsu Expert: Hotaru is a expert in using a Tantō, using in a form of a clover. He usually shows his skills by attacking Tanjiro after he damages his swords
- Hotaru's surname contains the kanji for "steel" (鋼 hagane?), a reference to his occupation, and his first name means "firefly" (螢 hotaru?).
- He hates his first name due to how "cute" it is.
- Hotaru's favourite food is mitarashi dango, and he goes out to buy some whenever he loses his temper.
- Hotaru has wind-chimes attached to his hat because he's liked them since he was a child. Whenever he would throw a tantrum, the adults would use the chimes to calm him down.
- As a child, Hotaru was so horrible to deal with that his parents had a mental breakdown and Tecchin Tecchikawahara had to raise him.
- Tickling his sides is the only way to break his concentration, after which he humorously goes limp for several minutes.
- In an omake, Hotaru is depicted as having a wife, but during the Swordsmith Village Arc he is shown to be single.