Final Fantasy XIII, divulged bits of information about the upcoming RPG in an interview with Famitsu PS3. We already know the demo is lengthy, but now we also know that it will allow players "to play from the opening through the prologue," similar to the Final Fantasy VII demo. Besides Lighting, players will also be able to play as the blonde male (shown above) in the demo. He remains nameless, but whatever his name is, it will be weather-related as well.
Toriyama then talks about a "new style" where FFXIII will not have an actual main character. Instead, players will experience the story from the viewpoints of different characters. One of the main focuses of the demo will be to allow players to try out this "new style." The battle system also allows you to switch characters; it will be party-based and the player will switch between each character during combat.
As a bonus, Forever Fantasy has scans of the screenshots from the FFXIII story. There are even a few Versus XIII pictures as well. Take a look here.