As far as Arata, I take some umbrage with the whole "singular traumatic event" aspect of his character. While I don't find it unrealistic, it strikes me as disingenuous and unrepresentative. A singular traumatic event is convenient in writing, but here it really feels like it's missing the point and misrepresenting causes. For instance, read here about the difference between Selective Mutism and Traumatic Mutism:
selectivemutismcenter.org/whatisselectivemutism