Roxas

Roxas is number 13 in organization XII. He is Sora's Nobody and looks exactly like Ven from Kingdom Hearts Birth By Sleep. There's a reason for this but would spoil Birth By Sleep.

Weapon
He uses The Keyblade.

In Kingdom Hearts 2 he uses Oathkeeper and Oblivion when you fight him in The World That Never Was.

Kingdom Hearts 358/2 Days
He is the main playable character in Kingdom Hearts 358/2 Days.