>'why does the character need to be autistic?'
Absolutely this. Forgive me for sounding a bit preachy on this, but I'll launch anyway. This is in no way intended to scold or criticize.
To me, a character in a story is first of all a character. They are there to serve the story. In that respect, they don't need to be anything in particular. They are the MC or the villain or the faithful companion and so on.
If I start with autism, I'm starting at the wrong end (with one exception, see below). I could start with a character who is fat or is Italian or is missing an arm. In every case, I've started with a stereotype, a cartoon rather than an individual, and certainly not a character who has a role in a story. Such a representation will almost always read hollow, shallow, one-dimensional, because I've started with a trait rather than with a person.
The exception to this is if my story is *about* being Italian, about being overweight, about being autistic. There's room for argument whether such a story really belongs in the fantasy genre or in literary realism, but that's a separate thread. Just in terms of character creation, it's important to know why each and every character is there in the story, what role they serve. Even the minor characters--I'd argue especially the minor characters, when it comes to making a fantasy world feel true.
For myself, here's a possible angle I might take. You can consider it or not.
Do some research on autism (you've done this). If there are aspects along the spectrum that seem to hold potential for my story, I'd look more deeply into those aspects--behaviors, difficulties, perceptions. Where it feels right, I'd incorporate those into a character and see how it played out in specific scenes.
As others have said, I would not use the terminology of autism (remember, I'm assuming we're not working on a book about autism, we're working on a book about dragons, or whatever ... although a dragon with ADD has potential ...). I would just write the character with these intriguing traits. If some readers interpret that as the character is autistic, that's fine. If it goes right by, that's fine too. Because the book isn't about autism.
Well, you did ask. That's me being brief. <g>