Synch Blade is an ability which allows a Keyblade wielder to summon another Keyblade during battle. So far, the only characters to use this ability are Roxas, who can use this ability to summon his two Keyblades, the Oathkeeper and Oblivion; Sora while in Valor Form, Master Form, and Final Form; and Riku.
Roxas can use Synch Blade because he has both Sora's Keyblade and Ventus's Keyblade, due to Ventus's heart remaining within him. Originally, he just wields Sora's Keyblade, as the creation of Xion had siphoned away Ventus's Keyblade.
However, when she is defeated, his powers return to him, and he reawakens the ability to use Ventus's Keyblade. After Sora and Roxas come back together, Sora is able to mimic Roxas's Synch Blade within several of his Drive Forms, but since Sora possesses every ability that Roxas had, it should also be possible for Sora to use Synch Blade naturally, even without going into a Drive Form.
At one point at The World That Never Was, Riku also demonstrates Synch Blade by wielding both his Way to the Dawn and the Destiny's Embrace, the latter of which he hands to Kairi. However, it is unknown how Riku obtained a second Keyblade.
Learning Synch Blade
Kingdom Hearts II and Kingdom Hearts II Final Mix
- Roxas learns Synch Blade immediately before fighting Axel in the Basement Hall. The ability can only be used during the fight.
- Valor Form has Synch Blade as a default ability.
- Master Form has Synch Blade as a default ability.
- Final Form has Synch Blade as a default ability.