EW has posted the first photo of actress Evangeline Lilly as Tauriel in "The Hobbit: The Desolation of Smaug". The "Lost" actress has also spoken a bit about how the character fits into the saga:
Tauriel is the head of the Elven Guard. She’s a Sylvan Elf, which means she’s of a much lower order than the elves we all became acquainted with in The Lord of the Rings. She doesn’t hold the same kind of status that Arwen or Galadriel or Elrond or Legolas do — she’s much more lowly. She sort of goes against the social order of the elves a little bit.
Tauriel’s relationship with Legolas is significant. They’ve known each other since they were children, and Legolas’ dad, [Elven king] Thrandruil, has a soft spot for Tauriel and sees something very special in her. So if you grow up side by side, and your dad has a very special spot in his heart for this young woman who’s a fantastic warrior, I think it’s hard not to notice her. That’s probably as much as I can say.