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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...

Here is the true answer.
The Blonde Haired Kid's name is Roxas, as my username implies.
He is Sora's "nobody". This is explained in the game.
He is a member of Organization XIII but has no memory...