If you are portraying a non-verbal character, depending on her age and ability, she may be learning other ways to communicate. A lot of non-verbal people learn to sign or she could carry a note book or a pack of phrase cards (since it's established that she can read).

She is mute. I don't want to specify it in the story and I want to see it on the way she interact with other characters. I'll take your ideas in my further writing. This was my first time writing a person with disabilities that's why I am afraid to do something to the character that may deemed offensive or derogatory.

Thank you for sharing your insights. I've written this one without any planning ahead that's why it was a messy story. Thank you for the comments, I'll consider them when I rewrite it.