His name is Hope Estheim. The white-haired boy did make an appearance in the E3 Final Fantasy XIII trailer, but thanks to this Japanese magazine article, we have more details about him.
Hope wields a boomerang as his weapon, and there is a bit of friction between the fiery tempered character and bandana dude Snow. In the FFXIII, Hope's mother falls from a ruined bridge.
So there you go.
Final Fantasy XIII Scan Brings New Character Details [Final Fantasy-XIII.net]