Shototello headcanons! —From an insta story, putting them here too :]
Donnie is absolutely an instigator. Both because he likes Shoto’s angrier side and because he thinks he could use the catharsis lol
Donnie once made a satellite that could snipe Endeavor dead (or like, stun gun him) from the stratosphere with the push of a button, and gave Shoto the only remote as a gift. He did not expect this to make Shoto as upset as it did. (The satellite is now [temporarily] disabled)
Though Donnie’s aversion to touch mostly comes from germaphobia, Shoto thinks it’s because he finds skin to skin contact overwhelming (when that’s only really the case when he’s already stressed). Because of this, he always wears his turtlenecks and long sleeves whenever he goes to see Donnie cause he thinks it gets him more hugs. (Remember when bro got turned into a koala? He is not beating the allegations)
Courting a romantic partner was the first time Shoto was ever really embarrassed or nervous to tell anyone he’s from a quirk marriage. When Donnie did eventually find out, he ended up making hoards of secret plans for many different ways to dispose of or —at the very least— beat Endeavor in a fight. Just in case. (I think thanks to Big Mama he’d be very familiar with the idea of a parental figure relapsing in their growth)
Spiny softshells are quite territorial, Shoto has a quite sizable fanclub in his second year onward. He is quite relieved that Donnie can ward them off and he is absolutely glad to do so.
Donnie told his brothers Shoto doesn’t speak english (incorrect, he does canonically know some english, they r both bilingual—) to try and keep them from embarrassing him. It backfired and ended up being the final straw for them to start trying to learn japanese. They now talk to Shoto more than they would if Donnie just told the truth from the beginning hahaha. (Shoto went along with it because why not, even though the secret came out eventually. Bro is having fun being a shenaniganizer for the first time in his life)
Donnie loves the challenge of making Shoto smile, it’s why he likes “cute and mean,” it feels like treasure of some kind.
Shoto always discourages Donnie from trying to kill or harm his father in any way, but he is also a little glad to have another friend like Inasa who is just like, “I straight up HATE your dad dude, if you want me to literally end him i’ll do it, I am waiting for the chance, I am a sleeper agent waiting for your command.”
Donnie once called him a "sweet chinchilla" (which actually did make him smile a bit somehow-) when April was in the room. The cringe of that nickname compounded on itself from her ancestral memories beamed across the multiverse (2012) and hit her with a cringe so hard, she immediately passed out upon hearing it. None of them know why the hell this happened, but for Shoto, it only confirmed a devastating blow; if he smiles, women will die.