SKIN.
as we all know , katsuki’s quirk is a combination of mitsuki’s and masaru’s quirk; mitsuki having the ability to produce glycerin & masaru having the ability to produce acidic sweat with combusting properties. their combined powers have forged an even more powerful quirk within their son & through daily training , he makes it more so every day. but i wanna talk about his skin for a second.
katsuki’s got some VERY nice skin , thanks to the quirk from his mother. mitsuki doesn’t look her age whatsoever , despite being in her late 30′s , because of the glycerin she produces. glycerin is a humectant & is used so much within the world of skin care because it helps the skin to retain moisture & keep it hydrated , relieve dryness & dullness , & can even even protect against harmful microorganisms because of its antimicrobial properties. on top of already having this unfair advantage for great skin , katsuki also takes extra steps to upkeep his skin , ranging from the normal washing every day to exfoliation twice a week & a face mask every two weeks. katsuki knows he’s got good skin & it’s something of a boost to his pride to make sure that his skin is taken care of. as soon as he gets roughed up through rigorous training , he takes the proper steps to help his skin return its former glory & producing glycerin speeds up that healing process ( as inrl glycerin does for things like scars , bumps , discoloration & more ).
on the flip side to that coin , there are also some downsides to having skin like that. with having smooth , soft skin most of the time comes EXTRA SENSITIVITY , since the surface of the skin isn’t calloused and protected by rougher patches. this probably irritates katsuki the most because as a person who runs his mouth all of the time & is constantly one upping the others around him , sometimes being too physically sensitive comes at a price , if ya’ get my drift. see: ticklishness. in addition to that , glycerin can actually also be harmful to the skin in certain situations. since glycerin is a humectant , it draws water from the nearest source. if the humidity levels in the area are too low , then using too much glycerin will make it start forcing water out from the lower levels of your skin. that causes dehydration , to the point where your skin could even begin to blister. because glycerin is produced all over katsuki’s body , it’s safe to say that if he went too ham with his quirk in a place where he shouldn’t , he could end up severely dehydrated to the point where it could become fatal.