Autistic Ushijima headcanons!!
Ushijima, other than Kageyama, didn't actually get diagnosed until he was out of highschool.
Not because he didn't show symptoms as a child, he absolutely did, but because his mother refused to have him diagnosed.
Until he was 5 years old he didn't speak. At all.
He did learn how to write and read by the time he was 4 but the most his family could get out of him in terms of vocal communication were some grunts and huffs and such.
He started speaking in time to enter elementary school but it was short sentences and very few of those.
He is extremely bad at masking. Try as he might, he has no idea how tge whole thing with facial expressions works. Or how to give his voice the right tone. As a result, his voice is usually flat and he has a pretty deadpan expression.
Ushijima isn't stupid but he does struggle with things that seem illogical to him. For example, he is great at math but English confused the hell out of him because it's a shit show of a language and half the words sound fake.
He has no idea how to hold a proper conversation. Talking to him is more like talking at him. He's listening (most of the time) and it's not like he doesn't care, he just doesn't know how to connect and answer to what you're telling him.
It's extremely rare for him to initiate a conversation, he usually only does this with teammates (mostly Tendou) or occasionally Oikawa. (This does get better during and after highschool but he still dislikes starting the conversation)
I think it's needless to say that volleyball is a special interest, that's pretty much all he cares about.
Now here's the kicker. Ushijima had NO IDEA that he's not neurotypical, his parents never told him anything like that! Pretty much the opposite. Whenever he'd bring up his teacher's concerns with his mother she'd tell him they're just being ridiculous.
The entire team in highschool knows. They just know. It's in the way he doesn't make eye contact, he sways back and forth when he stands, he cringes at certain textures to the point it causes him anxiety or even nausea. The way he drifts off during class, chewing on the back of his pencil, the way he stands with his back slightly hunched and his hands against his chest before he catches himself doing it and straightens up.
He suppresses his ticks because his mother taught him they're inappropriate. Even into adulthood whenever he gets the urge to flap his hands he slaps his own wrists instead and moves on.
He doesn't like being touched. It causes an unpleasant tingling on his skin and he'd rather not have people come too close to him.
something about advertisements catch his eye. There's nothing to read into, no emotional expressions he has to try and understand and no complicated storylines. He enjoys adverts.
Tendou I his comfort person. Now that doesn't mean that his boundaries when it comes to physical contact don't apply to him. It DOES mean, that he has an easier time talking with him and understanding him. Because Tendou over exaggerates his emotions and expressions in an almost theatrical way, making it easy for Ushijima to understand his moods.
Ushijima does not know when it is or isn't appropriate to speak. While he generally prefers to stay silent, sometimes he will blurt out with an unintentionally inappropriate statement in an equally inappropriate decision.
For this reason funerals, birthdays, wedding and other social events like that are extremely difficult for him to deal with.
even as an adult he deals with nonverbal episodes. He goes mute for different periods of time for different reasons. Sometimes it creeps up on him slowly and he knows it's coming and sometimes it just happens one second to the next. But it doesn't bother him much. The people he's close to can talk to him and he'll write or sign in reply.
He wears heavy clothes because they calm him. Leather boots with thick soles, heavy jackets and belts. He can't sleep without a weighed blanket.
Ushijima slowly learns to accept his disability after he gets diagnosed. His new teammates don't treat him like any less of a person because of it. Tendou and the rest of his old team are there with support and help wherever they can.
He eventually starts talking about it more openly and with that comes a certain bond to Kageyama.
Kageyama understands. Kageyama knows that the sounds and feelings connected to volleyball are good sensory input. Kageyama knows to stand back when Ushijima starts to get overwhelmed.
Kageyama brings Ushijima a small fidget toy, a Tangle, to practice one day because Ushijima has been struggling to find ways of stimming that make him comfortable.
For Ushijima it's a slow process. He still doesn't understand social cues or small talk or facial expressions. But he's managing.
[disclaimer: I am autistic, I relate to Ushijima in a lot of ways and this headcanon isn't supposed to be romanticising at all, I just want to share my thoughts]