Keyblade Armor

Armor, in terms of Kingdom Hearts Birth by Sleep, is a metallic bodysuit worn by Keyblade wielders Terra, Ventus, and Aqua as it protects them from the darkness as they travel to new worlds. Appearing on their bodies by touching the shoulder guards they wear on their arms, the armor appears as commanded to fulfill its purpose, much like the Keyblade does.

Kingdom Hearts Birth by Sleep
Terra, Ventus, and Aqua each wear their armor as they travel to other worlds, the armor protecting them from the great darkness. Seen many times throughout the storyline, it is also used for other defensive purposes, each of the three Keyblade wielders calling upon their armor before the final battle with Master Xehanort and Vanitas in the Keyblade Graveyard.

When Terra defeated Master Xehanort in their final battle, the elderly Keyblade master unlocked his heart. Terra attempting to protect himself from Xehanort with his armor, this proved to be of no avail, the man possessing him. Terra's armor fell to the ground as the new Xehanort walked off, Keyblade in hand, Xehanort shocked to see the armor reassembled and standing before him wielding Terra's Keyblade. This entity known as the Lingering Sentiment, it proceeded to battle Xehanort. Victorious, it disappeared after Xehanort vanished in a burst of light and was transported to Radiant Garden.

On a desperate search for Terra after sealing Ventus away in the newly formed Chamber of Waking, Aqua later found her old friend in Radiant Garden. Surprised to be attacked by him, she had no choice but to battle Terra, who she soon realized to be Xehanort. Terra's remaining consciousness within his body forcing Xehanort to unlock his own heart yet again, the body fell into the Darkness. Aqua then sacrificed her armor and Keyblade to save it.

In the Xehanort Reports, Master Xehanort wrote that he had once worn armor, but he ceased to do this when he felt "a strong presence".

Kingdom Hearts II Final Mix
New cutscenes made for Kingdom Hearts II Final Mix revealed that some time after Dilan carried Aqua's armor and Keyblade to Ansem the Wise's castle after the discovery of Xehanort, it was placed in the Chamber of Repose, where it was to remain. Eleven years later, Xehanort's Nobdoy, Xemnas, came before the armor, adressing it as his "friend" regardless of the fact that his complete being had lost all memories of the armor and who it belonged to. This aroused suspscion in Organization XIII, its members curious as to whom Xemnas was talking to and the importance this being had.

Terra's armor later appeared before Sora and friends as the Lingering Sentiment, mistaking Sora to be Xehanort and attacking him in a great rage. Upon its defeat, the Lingering Sentiment acknowledged Sora as a Keyblade master, disappearing soon after.

Trivia

 * In Kingdom Hearts II: Final Mix and Birth by Sleep, Terra, Aqua, and Ventus's Armor are adorned by capes. The capes were removed in Kingdom Hearts Birth by Sleep, however, for reasons unknown.