Famitsu.com has interviewed Final Fantasy Versus XIII director Tetsuya Nomura about all the projects he's involved and we have some specific details about Versus' main character. A couple of days ago we received information that the Prince was named Noctis. This has been confirmed by Nomura but apparently they will rarely be using this name in the game. They will be using his nickname "Noct" throughout the game. Thanks to our forum member Adria and her friend who translated it, we have some more details from this interview about his character.

Noct isn't an unsociable person but he is shy.
He usually smiles except for the party scene where he meets Stella. He tries to hide his shyness by acting in a calm, confident manner.
It's difficult to define his character. It's hard to tell what's really going on inside.

Source: Famitsu.com