So, some facts to consider:
-> Katsuki Bakugou sweats nitroglycerin from his palms
-> He sweats a lot of it, particularly when in combat/excising (see: the giant bracers he can fill on his arms).
-> Quirks often have negative side effects on their users, usually related to not being immune to some element of the quirk ex: being able to create fire but not being fireproof.
-> Nitroglycerin is used medicinally as a strong vasodilator (lowers blood pressure).
-> Nitroglycerin can lower blood pressure dangerously even at low dosages.
-> While people can build up some immunity to it over time, high exposure like Katsuki would have would still be dangerous. Unless his body had some sort of counter.
-> There’s lots of ways a body could raise blood pressure. One of them? Stress.
-> Stress (adrenaline and cortisol) can significantly raise blood pressure and do so fairly quickly.
My conclusion? Katsuki having No Chill Ever is actually his body’s way of keeping his blood pressure even. He’s constantly at 99/100 stress level for everything because of this. It’s why he does everything so intensely, he’s got near max levels of Adrenaline in him at all times.
Now whether or not this is canon or what canon intended is up in the air, but it’s now my personal canon. Ideal results of this:
-> Katsuki’s stress levels are discovered when he and his classmates/maybe teachers are exposed to a fear/panic quirk that shoots your adrenaline and cortisol through the roof. His classmates are sent into a panicked frenzy, running around and hiding. His teachers are either also panicked or just barely holding on to rational thought through many years of practice. Katsuki…. is just fine? Mostly he’s confused as to what the hell is wrong with everyone else. Whoever they’re fighting isn’t actually that tough without their quirk, so Katsuki takes ‘em down. Everyone is put through medical to determine what happened and why Katsuki wasn’t effected. They discover the quirk didn’t work because like, the quirk is made to take someone from 5-10% stress to a sudden 90-95%. Katsuki has a resting 96.7% or something. The extra 3.3% doesn’t change much.
-> They try to put him on anti-anxiety meds or something to help. He starts to calm down for the first time in his life which lasts all of 5 minutes before he passes out and almost fucking dies from the blood pressure drop.
-> “So you’re telling me being calm for once in my life literally almost fucking killed me?”
“Hang on, I need to call my mom. I have an argument to win.”
