Before we get to the answer there is a key spoiler from Final Fantasy XIII below. Just a heads up if you care about that kind of thing.
"Serah is Lightning’s young sister who turned into a crystal in an early point in Final Fantasy XIII and Lighting goes on a journey to rescue her," Toriyama summarized. "In XIII-2, it’s the other way around; the two characters switched places. Lightning still plays an important role in the story, but the main character you play as is Serah who is looking for Lightning."
We saw a scene with Lightning riding Odin and fighting Chaos Bahamut at E3. Even though she is in a different world, Lightning retained her strength from Final Fantasy XIII. While Toriyama wouldn’t give specifics he said, "Almost all of the main characters from Final Fantasy XIII will make an appearance in some way or another."
We’ll have more from the Final Fantasy XIII-2 development team later this week.