Square has announced DLC for Final Fantasy XIII-2 will be released next week and it allows players battle Lightning via the Lightning Coliseum battle.
If you beat her, she will join your party. Amoda will also be making an appearance, as you will note in the trailer (via Gematsu) posted below.
A second costume pack for Serah and an additional senario entry are in the works following the Lightning DLC, and a third coliseum battle will be released in mid-March.
The costume pack contains a bikini outfit called "Resort Style" and it will be released sometime this month. An Assassin’s Creed outfit for Noel has been announced, but a release date for the Ezio costume or Serah’s AKB48 outfit have yet to be announced, according to Siliconera.
A decorative monster accessory called Stilzkin’s Journal was released in Japan through an issue of Famitsu, but it has yet to be announced for westerners.
FFXIII-2 is out in the US now, and hits the UK tomorrow.