The more this thread goes on, the more I think we'll never know why Misha is at Yamaku. Yup, even after the release.
Anyway, I'd like to try to sum up everything, that might be of help.
What the devs tell us:
-During the ManiFest podcast, it is said that they don't have a clue about it, just that Misha was put there to make Shizune someone Hisao could communicate with. (I'm too lazy to listen to it again, if someone does, please confirm.)
-On an older blog post,
that one actually, Aura tells us about "missing the real hints". So, have they decided that she HAS a disability?
Ok, so listening to the devs won't help. (Though the "broke a nail in third grade" idea is really funny)
The symptoms
-General clumsyness. The stairs making her dizzy might be some lie to trick Hisao, however I don't think she lies about her aim during the festival. Plus, in the scene where they order chinese food, Hisao notices that she has trouble dialing the number, and she says that she has trouble breaking chopsticks. Is her clumsyness a symptome, or just part of her personality? Compared to other "clues", it's still the most serious one.
-Being loud: I don't think I need to sum it up, she definitely has volume issues. But again, is it really related to her disease, or is she that way just for fun? I think she was made that way just for fun.
-Knowing sign language: I don't think it might be a symptom. She may have learnt a bit of it by coincidence before she met Shizune, and that's it.
Now, at this point I would have just told my theory (Some kind of nerve disease), but after all i've written, i'm doubting myself. Too bad. So i'll just post all of this.
However, if the devs REALLY thought of a disease for Misha, I don't think it would be a rare/obscure illness or a mental one. First, that would be extra work for the devs if it was an obscure one, and as for a mental disorder... I don't know, it just doesn't suit her, I guess.
Have fun trying to figure it out, but I have the feeling that her disease is not one you need to check wikipedia five times to find it. Just a feeling though.