Arwen, also known as "Undómiel (Evenstar)", is a half-elven from J.R.R. Tolkien's The Lord of the Rings. She is the daughter of Elrond and eventually the wife of Aragorn. She is beautiful, gentle, and wise, or at least intelligent, as befits an Elf. While her character is not given as much in-depth description as some of the other members of the cast of LotR, she is definitely a sympathetic character. Which is why the way she is characterised in badfic is so inexplicable.
In badfic, Arwen is either competing with the Sue for Aragorn's manly affections, or braiding her hair and twittering about how wonderful Aragorn is, because the Sue is going after Legolas. If the Sue is a Warrior! or Tomboy! Sue, Arwen might also force (or be screamed at by the Sue for failing to force) the Sue into a dress or gown of some type, usually highly beribboned and often pink, despite the fact that in neither book or movie did Arwen (or any other female character) wear anything remotely similar.
If Arwen appears in m/m slash, it is almost exclusively because one of the protagonists is Aragorn, and he is trying to escape her evil bitchy clutches. If the fic is femmeslash, then Arwen is usually slashed with Eowyn, Galadriel (who is her grandmother, not that the badslashers a) know that, or b) give a damn) or Rosie Cotton. All three of these pairings are bizarre and extremely unlikely, but femmeslashers in the Tolkien Fandom don't exactly have a lot of choice.