(Spoilers included, information 70%-80% accurate, i still have yet to play the game)
Among fans of the KH series he was referred to as the BHK for the past 2 years, the game was released in japan a week or so ago and slightly before that his name was finally revealed to us, his name is an anagram of Sora (minus the X)
He is a member of Organization XIII, all members have an X in their name, which can also show how Roxas relates to Sora and the Organisation just through his name. He is a Keyblade wielder, and his ability to use the keyblade was givin to him when Sora temporairly became a Heartless in KH1. Roxas is the thirteenth member of the order and is also featured in the 'Deep Dive' movie shown at the end of KH: Final Mix, he is shown wielding two keyblades, the Oblivion and Oathkeeper, after fighting the heartless, he then rushes up the side of a building where he is aided by Riku, who after all the commotion then turns on Roxas and knocks him out.
The first member of the organization 'Xemnas' reveals Sora's existence to Roxas, causing him to leave the Organization
Roxas' memory was altered by Namine and he now resides in Twilight town, unable to recall his memories of being with the Organization.
Roxas' face was first revealed at the end of KH: Chain of memories, he was also suggested as the other side of Sora's heart in this game.
Roxas is the character you play as for the start of the game in KH2 and also an important character throughout the rest of the game.