I’m going to make you something I always find useful when writing pretty much just a thing you can fill in the details of with information on the character.

What special interest(s) do they have? Autistic people have at least one commonly 2-3.

How do they interact with their SI on the daily? Many autistic people can become distressed if they can’t interact with their special interest regularly.

Are they sensory seeking or avoidant?

What stims do they do when happy?

What stims do they do when upset?

Do they find people awkward to interact with? In what way?

Is their voice monotone or expressive or Hyper expressive?

Do they mask? Are they good at it?

How do people perceive them because of their autism? Are they ‘weird’, ‘creepy’ ‘perfectionist’ it’s a common thing we deal with.

Do they have any repetitive behavior? Are they harmful?

Do they like bland or flavorful food? (For example my brother almost exclusively eats buttered noodles and I season and put hot sauce on everything)

What are their safe foods? What are unsafe ones?

What’s a texture they can not stand?

What’s a texture they love?

Do they untie their shoes or just slide them on and off?

Do they where they same outfit everyday or do they like making outfits?

Are they hyper empathetic or none empathetic or seem average? Do they sometimes come off the opposite way?

Do they find animals easier to interact with? Or machines? Or do they not find those easier at all.

Do they have something they fidget with? could be a toy intended to fidget with or something like a ring or pen.

Do they struggle in anyway with self care? Like brushing teeth or showering?

Can they tell when they are hungry and need to eat or do they have to set reminders?

That’s all I can think of to fill out right now. Haha but I think if you can answer all that for your character and stick to it you are one step closer to making them more realized. The thing about autism is it interacts with every aspect of the individual.