I've seen plenty of people assume that the girl was deaf just because she didn't speak much. I didn't read it that way. If she was deaf then why would she go into the surf shop to talk about the prices, while he waited outside?
But if she was not deaf, why did she allow Shigeru to miss his turn at the surfing competition?
Also, she's got a flashing rotating light thing in her house to alert her when there's someone at the door - surely that's a specialist piece of kit for the hearing-impaired?
I think she's just better at communication, with gestures and hand signs, than he is, so she tries to sweet-talk the surf shop people. She blags a lift to the competition, too. And in the short scene at the end, when she gets a lift again from the same guy, they are riding in silence, she doesn't thank him or anything.
A beautiful, thoughtful film, from an amazing director.