WARNING: The following contains spoilers from the Demon Slayer: Kimetsu no Yaiba manga, by Koyoharu Gotouge, available in English through VIZ Media.
From his outfit to his Breathing Style, there is no one else quite like Inosuke in all of Demon Slayer. The savage fighter with a heart of gold is most famous for the boar's mask he wears on his head. The mask is of high sentimental value to the wild demon slayer, as it's actually the head of the boar that raised him in the mountains. However, the mask may have an actual purpose that contributes to why Insouke is seldom seen without it.
Inosuke grew up alone in the wilderness after his mother dropped him off a cliff as an infant, a last-ditch effort to save him from Doma, Upper Rank Two of the Twelve Kizuki. After killing Inosuke's mother, Doma assumed that Inosuke had perished from the fall and returned to his lair. Inosuke was carried downstream and was eventually taken in by a wild boar, who raised him as its own until it passed away.
As a tribute to the only mother he ever knew, Inosuke began wearing her head as a mask. He rarely takes it off, as it seems to be something of a security blanket to him. When he, Tanjiro and Zenitsu took refuge after their first battle together, Inosuke took the mask off while recovering but then put it back on his head when it was time to sleep. When he met Doma again during the Infinity Castle Arc, Inosuke was irate when Doma forcibly took the mask from him.
As if a feral demon slayer who chips his katanas and wears a boar's head as a mask wasn't unusual enough, Inosuke also has a very feminine face beneath his mask. After initially mistaking him for a monster with the mask on, Zenitsu then mistakes Inosuke for a girl when the mask falls off. Even Tanjiro comments that Inosuke's face is "very petite and fair-skinned and therefore attractive."
Inosuke doesn’t seem to have an opinion regarding his face, but his appearance might be a factor in his refusal to take off the mask in most circumstances. Growing up in the wilderness means survival of the fittest, and Tanjiro notes that Insouke fights like a wild animal. It's possible that part of the reason he wears the mask is to be more intimidating, considering he spent his youth in the wilderness fighting other animals, which he considers the one true joy in life.
Inosuke's fighting style and even his social interactions are similar to that of a wild animal. In his first few days with Tanjiro and Zenitsu, he is constantly picking fights over things like food and sleeping arrangements. These animalistic attempts at dominance all fall flat in part because of Tanjiro's kindness, which clearly frustrates Inosuke, but also because he could not be less threatening without his mask on. Even Zenitsu becomes much less afraid of the Beast Breathing swordsman after finding out how pretty he is.
