A first glance at Kirishima’s hero costume a person would probably question why are they wearing such a strange get up, but what may seem illogical for the onlooker is a logical decssion by the wearer. Kirishima’s quirk is hardening and when he use’s it his skin hardens and expands and if he’s wearing any clothing they will invebilty rip as demonstrated by the pictures featured here.
While hardening may not be flashy it’s weaponary and armour apllications are useful in that it is a effective offensives and defensive weapon. It protects him and damages the enemies he comes in contact with, but when he’s covered in a fabric it becomes a less than effective weapon for him to use as he is constricted because of the lesser movement given to him as mentioned before with the expanding applications of his quirk he would fill out his shirt quickly and if you have ever tried to punch a guy in a too tight top you would expend more effort than needed as you are also breaking your top. There is reason a majority of superhero costumes are made out of spandex it’s for the reach.
As noted by the author about Kirishima’s outfit, it is made abuntley clear that the clothing that he wears when he hardens his body will get shredded. Which is why he is topless and why is pants are so baggy it’s to prevent ripping and being impeded in battle but in contrast his legs are not readily available for battle as the other half of his body is, but as he use’s his upper body far more than his lower body in his fighting style this makes sense as I have always’s seen him punching objects rather than kicking. Later in the story he has a questionable new fashion choice that when I first saw I couldn’t stop laughing but after I got a few chuckles out of my system I realised that his outfit has a reason to be like this.
As you can clearly see in the pictures I have allocated he makes a fist to harden his hand but his sleeve remains intact so his ability as a hero isn’t impeded by a shirt which would be destroyed anyway and by allocating hardening to his fists and torso he isn’t limiting himself rather it’s a controlled use of his quirk. When not hardening the entirety of his body but rather he can concentrate on smaller areas so that they remain harder for longer and as there aren’t vital organs in his arms he doen’t need to concentrate in these areas and even if he has to for certain reason the material is very durable and would be able to withstand the bare need for him to use his quirk, as mentioned in the author’s notes about this costume the civillians complained that the hardening hurt them and so Kirishima implemented this new design although in his unbreakable form it breaks away compltley and even in the inbetween forms it has been shown to break. With only leaving his vital organs hardened while leaving his less than vital parts open for attack this means his thirst for battle shall rage longer.
Kirishima walks around topless because he understands what the applications and the restrictions of his quirk is and is using it in a effective method that complements his fighting style rather than restricts it. Though the pin wheel in his initial design makes no sense for combat it’s just their to make him look manly.