It is a very common thing as Judy said. Unless your child is having other problems, or symptoms of stress or anxiety, then I'd just let her be.
My older sister had an imaginary friend, Merker. When my sister did something wrong, she would say "Merker broke it", and it was impossible to get mad at her apparently.

She had a little imaginary friend, and nothing harmful ever came of it.
If you think it could be something more paranormal, see what your research shows. I had an experience a few months ago visiting a friend in her apartment. I had a vision of a child falling out of a window. Turns out, it was true after a friend of a friend was old enough to remember the story.
But, either way, if she's happy and content, I'd just observe, and let it go.