replying with a comment because its easier than tags but holy moly ! autistic bkdk are so important to me lol
easy and top of the list: quirk/heroes as special interest with a potential of hyperfixation/hyperfocus of them to a certain degree. izukus in autism heaven in his room lol. infodump KING.
stims by clicking pens, pulling/biting his lip, various hand gestures/arm movements. also doodling. he loves tangles because it helps keeps his fingers from locking up while writing and chew necklaces because he ends up hurting his lips and nails a lot lol
idk i personally think hes really really really picky about his food being served a specific way or only eating like 5 foods ever due to sensory issues. he has that vibe about him.
really bad at processing and displaying his own emotions/distress but has hyper-empathy towards others!!!!! this was awful in grade school when the "weird kid" was crying over your scraped knee more than you were lol.
because of that if hes verging a meltdown or sensory overload he refuses to tell anyone and hides. not cool izuku!!!!
easily embaressed about showing signs of his autism so he either masks it as his personality (e.g explosion stims, echolalia, audio processing delay) or hides it all together (special interest — for izuku, his parents and inkos eyes ONLY!!!)
as previously stated i believe he likes to stim with clenching his hand (e.g using his quirk) and letting off sparkler-sized explosions.
i also think he likes small fidget toys he can hide in his pocket. echolalia is 'extras' and 'hah?!' because he likes how they sound lol. audio-processing is terrible for him so he calls everyone weird nicknames until he actually learns their name.
he has extremely low-empathy vs izukus hyper-empathy. also terrible in social situations/reading the room.
his special interest of heroes rivals izukus, but because he double-majors special interests with various chemical/science related things, he lacks the extreme hyperfixation/hyperfocus that happens for it.
autism denier #1 it took him forever to be comfortable with his diagnosis tbh. its why hes so secretive and cagey about it, except for select people.
hates hates hates anything around his neck/choking him. why he doesnt wear a tie. i think of it as a wombo combo of autism+sludge trauma.
they have a ritual they do daily where every few hours, they share one fact from their special interest/hyperfixation at the moment. izuku always breaks the rules with 6 paragraphs and katsuki once spent 3 months only sending audio recording of his sick live drums from his new drumset.
misuki is a great mom, but so loud, so inkos house is sensory heaven for katsuki. shes always keeping low lights, quiet voice, and izukus fidget toys and various weighted items are everywhere. katsuki loves decompressing here and inko just makes him tea and some cinnamon-based treats. he has his own key lol. izuku finds it cute when katsukis all curled up on his couch with izukus handmade weighted throw.
visa versa, izuku loves katsukis house. mitsuki is the only one who can give him haircuts. masaru has a huge reference library so izuku love reading in their house, especially their books on fashion in the hero idustry. katsuki sometimes comes home from training to see izuku reading a book while upside down on the couch.
katsuki is FINE TUNED in izukunese so he ends up getting a 6th sense for pre-meltdown or pre-overload izuku based on body movements and vibes only.
katsuki is the straight man of the two until izuku gets so offended by someones lack of all might knowledge, katsuki has to step in with the infodump. they genuinely share one braincell 💥