I think Lightning, but my guess is that "if" she's will be added as Dlc, it will not be before more information or a trailer is revelead for Final Fantasy 13-3. Superbot went with the new(est) designs of the 3rd party characters (except Big Daddy) so they might do the same with Lightning. :S
Apparently they're going to have a conference in January to talk about 13-3 (and hopefully Versus XIII, found this information on their board the other day) so it wouldn't be too long after Kat and Emmett come out that we'll see some new DLC news IF she's to be put in.
I'd rather have Cloud though, or Raziel or Kain from Soul Reaver.