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From your description, I'm not so sure that there's a mental health issue involved, but perhaps if a school psychologist or guidance counselor observed her, he/she could probably get a good idea.
My thought though is her home life and other relationships. Does she have a lot of friends? Does she interact with the students? What does her family seem like? Do her parents work a lot? Is she an only child? Does she have sibilings somewhat close in age? These are all questions to explore because maybe she isn't getting enough social interaction with people, yet she really wants this time so she talks to herself or an "invisible person." That's just a thought. Try to engage her more with classmates (like at recess, etc.) when at all possible, especially the ones who you know will talk to her. That way, she will have someone to interact with. That's my gut feeling here, but I don't have this student nor do I interact with her on a daily basis, so your gut feeling is better than mine!
