In the 'Emily Rose Story', Emily Rose is a girl who has to endure the horror of being possessed. She is the one that everyone in the story is either trying to help or figure out what's wrong with. Her experiences are what drive the whole story forward.
The story of The Exorcism of Emily Rose has some basis in truth. But keep in mind that in the process of making it into a film, changes and additions were made to make it more engaging and cinematic.
Some elements might be based on real events, but it's not a completely true story. It's more of a fictionalized account with inspiration from various sources.