Arwen?
 

In the book Arwen has a small role, however in the film shes to play a much bigger role - from what I've seen of it anyway

Yep, they're beefing up the love story with her and Aragorn, and replacing Glorfindel with her. She actually carries Frodo on her horse to the ford, instead of Frodo just using Glorfindel's horse.

I think it's good that Arwen's role is expanded in the movie. There are so few female characters in the book. Thank goodness she will do more than be pretty and stay behind to sew a flag for her boyfriend.

They are taking some of it from appendix A in the back of The Return of the King. That's where the story of Arwen and Aragorn is. When RotK comes out viewers will know why they are getting married instead of sitting there wondering where that storyline came from.