At the Premier event, the XIII-2 unveil was preceded by an animated account of the Fabula Nova Crystalls mythology. This mythology serves as the foundation for all the games in the series, including XIII, Versus XIII, XIII-2 and Type-0 (formerly known as Agito XIII).
The purpose for showing this footage, explained Kitase to Dengeki, was to show that FFXIII only dealt with a portion of the mythology, covering such concepts as the fal'Cie and l'Cie. FFXIII did not explain the full breadth of the mythology, however. To better know the whole of the mythology, players will have access to the upcoming Fabula series titles.
XIII-2 covers a different area of the Fabula mythology from XIII. While the original focused on the relation betwen the Pulse and Lindzei deities, the focus for XIII-2 will be on the Etro deity. Toriyama said the game will delve deeper into the Fabula mythology this time.
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According to Toriyama, the XIII-2 story will be like taking the afterstory that was written in the bonus booklet that was included with Xbox 360's Final Fantasy XIII and expanding from there. The story kicks of after XIII, in the Gran Pulse world.
The story is a true sequel to XIII and will answer some of the mysteries of what happened in the background in the build up to FFXIII's ending. The world will expand further in XIII-2 (I'm not sure if Toriyama is speaking literally or figuratively here). The story's taste will be darker and more mysterious than the original.
Similar to XIII, Lightning will be a principal character. Other characters will also appear in the game in "unexpected forms," according to Toriyama. Some of the returning characters will see major changes to their atmosphere. The game will also have a number of new characters, in addition to the mysterious male character from the trailer.
That mystery character is the character next to Lightning in the game's logo, Kitase confirmed. He's meant to be a character in conflict with Lightning, a character of equal strength.
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The early trailer and single official screenshot show a major change for Lightning: she wears armor. While this is a big change, the character's image will not have a Final Fantasy X-2-style change, said Kitase. For those who haven't played it, FFX-2 had a poppy, comedic feel compared to FFX, with Yuna and the other leads dancing around and changing clothes during battle. XIII-2 will have the serious feel of a true numbered entry in the series, said Kitase.
Lightning appeared in the trailer surrounded by feathers at some points. These features are used in a number of areas, including parts of Lightning's armor. Remember how roses appeared on the screen when Lightning summoned Odin in FFXIII? This time, the feathers are meant to represent something holier -- a closer approach to the realm of gods.
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The two only briefly discussed gameplay. The game's battle system will be a further evolution of FFXIII's ATB (active time battle) system. Because it's a sequel, explained Toriyama, they want to actively use the the experience they gained in making the original to address issues that need to be addressed, and to make areas that are already considered good even better. For XIII-2's development, they've been running tests and listening to opinions from inside and outside the company and even from overseas.
Toriyama wants to add some sort of system for controlling difficulty. This could take the form of the inclusion of an easy mode, which is something they're currently looking into. Toriyama also assured that they're working to make it so that those who didn't play the original will be able to enjoy themselves with XIII-2.
Famitsu will be having its own look at XIII-2 later this week, so it's possible that we'll get even more details.
I seriously cannot believe it took me this long to say this. 
