I'm an adult and I have an imaginary friend
What do they look like?
Usually my imaginary friend manifests itself in the form of a freckle.
It's okay for people to use their imaginations and pretend that they have an imaginary friend. As long as you aren't delusional and realize on some level that your imaginary friend isn't really there, it shouldn't be a problem for you to have an imaginary friend since it doesn't affect anyone else.
I think doing this is childish, and I want to stop.
It is not common as an adult and if you want to stop having one, you should seek some professional help so that they can help you with the situation.