I'm going to tell my friend that many of the fictional characters she believes are real don't actually exist
Why do you want to do that to your friend?
I want to see her reaction
While it's good to be honest with your friends, it's best to always ensure your intentions are good. Although, you don't want your friend to live in a fictional world, it's good to respect how she feels.
I want to upset and hurt her because she is a b****.
Instead of trying to hurt your friend, it would be good to put yourself in her shoes to understand how she's going to feel after hearing what you have to say. Being a good friend requires you to be supportive and not rude to your friends.