The blond hair kid's name is Roxas.
It's an anagram of Sora's name with an X.
Technically, he is Sora because he is Sora's "nobody"
This is the only reason why he wields the keyblade.
Why is he wearing the Organization jacket?
I dont know but he used to be part of it but he
lost his memory.
If you played KH:CoM, if you finish Riku's Reverse/Rebirth, you will see a picture of
Roxas on top of Twilight Town's highest building.