Final Fantasy XIII breaks from series tradition is many ways, but one could make an argument that the way summons are handled is the most surprising.
As revealed by Square Enix staff during interview sessions at E3, you don't just summon FFXIII summons. You ride them!
Snow's summon was previously announced to be Shiva. The FFXIII version of Shiva differs from past Shivas, though, in that she's made up of two girls, one with a white face, and one with a black face. The latest issue of Jump has provided us with their names.
The one with the black face is Styria. She's the older of the two. The one with the white face is Nix (possibly named after the IGN writer -- we're not sure). Nix is the younger of the two.
You might have noticed this in the extended E3 trailer, but when Snow calls Styria and Nix out, they can either fight alongside him like a normal summon, or merge into a motorcycle for Snow to ride. When riding Shiva, the battle system switches to "Driving Mode." Subsequent attacks require that you press special button combinations to perform such moves as "Auto Drive" (Circle), Wheelie Rush (Up + Circle), Air Edge (X) and Diamond Dust (Triangle). This phase of the battle appears to be time constrained.
There's still a lot of mystery surrounding the game's Driving Mode. Will you get to ride everyone? If so, what will Lightning's summon Odin, shown in playable form at the Microsoft press conference, transform into?
As revealed during interviews at E3, FFXIII is 60% complete and is gearing up for a Japanese release this Winter with a world release to follow in the Spring. Expect details to start flowing shortly.