Tolkien's purpose in creating the character of Arwen

Arwen is signified by her sobriquet "Undómiel" which means "Evenstar".

Arwen (literally, "noble maiden") is the last brilliant embodiment of Elvish beauty and grace in the waning years of their relevance in the Third Age of Middle Earth. Arwen is the sixth and last elven maid to marry a mortal man (after Luthien & Beren, Idril & Tuor, Nimloth & Dior, Elwing & Eärendil and Mithrellas & Imrazór). In plighting her troth with Aragorn on the mound of Cerin Amroth in the forest of Lorien, Arwen renews the ancient alliance between the two races of the Children of Ilúvatar.

By marrying a mortal Man and choosing mortality herself, Arwen symbolically represents the passing of the torch from Elves -- and, indeed all the elder races (e.g., Dwarves, Ents) -- to Men in the Fourth Age. Arwen's death, one year after Aragorn Elessar's passing, embodies the diminishment of Elvish influence within Middle Earth, the dominance of Men and the dwindling and withdrawal of non-human races.

Arwen was the Evenstar of her people, the most beautiful, doomed shining figure within the gloaming of the elvish race. More than any other figure except perhaps Nienor, Arwen embodies the bittersweet, ultimately tragic melancholia that pervades J.R.R. Tolkien's magnificent cosmology. There is no ultimate triumph, youth and joy give way to old age and regret, and the nobility and grandeur of the elder races are gradually replaced by the relatively crass, graceless and violent dominance of humanity.