Thick, from the "Tawny Man" trilogy by Robin Hobb. Interesting one, as he's not remotely "high-functioning" in most respects. Thick, a servant, is seen by most people in the castle as a useless idiot, hence the insulting name. He has had a brutally hard life and can barely talk. But he is also prodigiously strong at the Skill, a form of magic that resembles telepathy. Fitz, another Skill-user, spends a lot of time figuring out how to communicate with him, and describes Thick as "not stupid, just different." Fitz is briefly shown how things look from Thick's point of view. It's a constant flood of sensory input: he makes little distinction between important things going on right in front of him and the slightest details in the background. At the same time, his mind seems to be constantly organising everything according to some grand abstract scheme. It's implied that his mental differences are actually an advantage in the Skill, though partly offset by his language problems.
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You're so vain
I bet you think this sig is about you