Roxas?!

Lets see, from the top of my head as the night becomes late here...

Roxas.
>Sora's Nobody.
>Created when Sora stabbed himself and became a Shadow Heartless.
>Because he is Sora's "other" he can wield a Keyblade and even dual wield with the Oblivion and Oathkeeper (some one will have to fill me in on why, I think it has something to do with memories).
>He strikingly resembles Ventus, I don't know why (maybe some can fill me here as well).
>He is the 13th Member of Organization XIII.
>In his Story, he runs away from Organization XIII (I think) and then gets captured by Riku to be used to help wake up Sora thus the beginning of KH2 when you start playing as Roxas.

 

Ok,Roxas can wield the keyblade because he is Sora's nobody.He is also a specail nobody,somehow he has real emotions unlike other nobodies.Not sure why he resembles Ven yet,will probably be explained. (see spam louge for Kupo's idea on that).He leaves the Organization after the events in 358/2 days to find out more about himself,but then get into a fight with Riku who he beats,but Riku takes Xehanort's heartless's form to defeat him.He is then sent into the digital Twilight Town until he is needed for an unspecified amount of time. (6 days?)

 

Oh right,one more thing.Roxas is merged with Sora,but he does seem to still have his own mind.Possible plot point there.

 

Well it sounds like the other guys pretty much explained the basics, but I guess I could throw in that he's the character fighting Riku in the secret vid "Another Side, Another Story" AKA Deep Dive.