Keyblade Armor

Armor is armor that protects the Keyblade Wielder from Darkness when traveling to other worlds. The Armour obeys their Keyblade Wielder just like the Keyblade.

So far, only Terra, Aqua, And Ventus have displayed the ability to call armor. Terra, Aqua and Ventus call their armor by touching the piece of armor on their arms.

Kingdom Hearts Birth by Sleep
Terra, Ventus, and Aqua all use Armor when they travel to other worlds. They ride their Keyblades while in their armor. Sometimes, they use it in actual combat, for example, when they fought Master Xehanort at the Keyblade Graveyard.

When Terra is possessed by Master Xehanort in Keyblade Graveyard, Terra's Armor come to life to become the Lingering Sentiment to defeat Xehanort.

When Aqua fights Xehanort in Radiant Garden, he unlocks his own heart and is engulfed by darkness. When Aqua and Terra/Xehanort are about to fall into darkness together, she commands her Armor to save Terra/Xehanort from swallowing by the darkness.

In the Xehanort Reports, Master Xehanort writes that he had once used Armor, but stopped, when he felt "a strong presence".

Kingdom Hearts II Final Mix
Aqua's Armor makes an appearence in the Chamber of Repose, and Xemnas addresses it as "friend". Terra's armor also appears, only this time as the Lingering Sentiment, an optional boss for Sora.

Trivia

 * In Kingdom Hearts II: Final Mix and Birth by Sleep, Terra, Aqua, and Ventus's Armor make an appearance with capes. The capes were removed in Kingdom Hearts Birth by Sleep, though.