As told in "The Tale of Aragorn and Arwen" Aragorn in his twentieth year met Arwen for the first time in Rivendell, where he lived under Elrond's protection. Arwen, then over 2,700 years old, had recently returned to her father's home after living for a while with her grandmother Lady Galadriel in Lórien. Aragorn fell in love with Arwen at first sight. Some thirty years later, the two were reunited in Lórien. Arwen reciprocated Aragorn's love, and on the mound of Cerin Amroth they committed themselves to marry one another.
In the first film, Arwen searches for Aragorn and single-handedly rescues Frodo Baggins from the Black Riders at Bruinen, thwarting them with a sudden flood, summoned by an incantation. In the book, Glorfindel had been sent by Elrond to look for the Hobbits and finds them with Aragorn. Glorfindel put Frodo on his own horse and sent him alone across the river to flee the Black Riders, for Elrond had pre-arranged for the river to flood when the Nazgûl entered the water. In the movie Arwen bears Frodo on her own horse across the river, driving the Nazgûl onward with her challenge. During this flight Arwen wields the sword Hadhafang, which according to film merchandise was once wielded by her father. This sword actually belonged to Idril Celebrindal, Arwen's great-grandmother.
In the The Two Towers, the injured Aragorn is revived by a dream of Arwen, who kisses him and asks the Valar to protect him.
Throughout the War of the Ring, Elrond begs her to accompany her kin to the Undying Lands because he does not wish to see another of his family die. Elrond shows her a vision of her long depressing life after Aragorn's death, and tells her that only death awaits her in Middle-earth. Arwen reluctantly departs for Valinor. However, on the road to the Grey Havens she has a vision of her future son, Eldarion, which belies her father's one-sided prophesy. She returns to Rivendell, and for her love of Aragorn refuses thereafter to leave Middle-earth.
In the The Return of the King, Arwen convinces her father to reforge the sword Narsil for Aragorn so that he can reclaim the throne of the King. Elrond initially refuses, but when Arwen begins to fall ill through her loss of immortality, he reluctantly agrees. Elrond takes Narsil, reforged as Andúril, to Aragorn at Dunharrow, and tells him that her fate has become bound to the One Ring, and that she is dying. How this came to be is left unexplained. In the extended version of The Return of the King, Sauron (through a palantír) shows Aragorn a dying Arwen in order to dissuade him from battle. The movies portray her as becoming human through her love for Aragorn; as in the book, Arwen follows the choice of her ancestor Lúthien to become a mortal woman for the love of a mortal man. She has a extended appearence in the movies rather than the books where she is barely mentioned. Sge appears only briefly in the main narrative with the majority of her and Aragorn's story relegated to a chapter in the appendix. In Peter Jackson's version, she is featured prominently in all three films and replaces several minor characters, most notably Glorfindel who aids Aragorn and the Hobbits as they flee the Ringwraiths. Not only does Arwen replace Glorfindel, she's given the additional task of personally carrying Frodo upon her horse, and her moment of glory — "If you want him, come and claim him!" — is a unique addition to the film. In the novel, Glorfindel places Frodo on his horse and then stays behind with Aragorn and the Hobbits to do what he can to slow the Ringwraiths' pursuit, at which point the narrative focus shifts entirely to Frodo