It may come as a surprise to some that Arwen played a much smaller role in the The Lord of the Rings novel. After Frodo’s injury at Weathertop, it is not her that comes to the group’s aid, but Glorfindel, another elf of Rivendell.
When Strider leads the hobbits away from Weathertop in hopes of reaching the Ford of Bruinen before the Nazgúl, they meet a golden-haired elf riding along the rode: Glorfindel. He heard of the passage of the hobbits through the wilderness by means of messages sent by the company of elves they met while still in the Shire. Fearing for Frodo’s injury, he sets the hobbit on his white horse, Asfaloth.
The four hobbits and Strider travel for a while with Glorfindel towards the Ford and the sequence plays out similarly to that in the film. When they draw near to the Ford, Glorfindel tells Asfaloth to run ahead, sensing the Ring-wraiths nearby. Frodo rides alone on the horse and narrowly escapes the Black Riders, crossing the Ford and witnessing the enchanted flood conjured by Elrond and Gandalf. Glorfindel sits at a place of honor at the subsequent feast in Rivendell celebrating the victory of the Ford.
There is also, in the depths of the mythology, another elf named Glorfindel. He is found in the tale The Fall of Gondolin. I’ll talk of this story in a later post more in-depth, but I mention it here because it is said that Tolkien considered making Glorfindel of Rivendell a reincarnation of the older one, as Glorfindel of Gondolin died while fighting a balrog. Tolkien passed away before this decision was ever finalized and the two are considered different characters today.
Glorfindel’s role in book is indeed small and does not go much further than the Council of Elrond at Rivendell, therefore I think Peter Jackson is justified in replacing the character with Arwen. He chose to give her a larger role and found an appropriate of a spot as any to include her, in my opinion. Tolkien was often criticized for not having strong female characters in his work (personally I feel like this is simply not true, but i can see where some would think that) and perhaps Jackson wanted to avoid such a charge.