Keyblade Armor

Armor, in the 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 as well as allowing them to breathe in space. 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
According to his Reports, Master Xehanort wrote that he had once worn Armor, but he ceased to do this when he felt "a strong presence".

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 attempted to protect himself from Xehanort with his armor, this proved to be of no avail as Xehanort possessed Terra anyway. Terra's armor fell to the ground as the new younger Xehanort walked off. With his original being's Keyblade in his hand, Xehanort was shocked to see Terra's reassembled armor standing before him wielding his Keyblade and proceeded to battle Xehanort. Victorious, it remained in the Keyblade Graveyard after Xehanort vanished in a burst of light and was transported to Radiant Garden.

After placing Ventus in the newly formed Chamber of Waking (and, by extension, Castle Oblivion), Aqua went to search for Terra and later found her old friend in Radiant Garden. Surprised to be attacked by him, she had no choice but to battle Terra/Xehanort. Terra's remaining consciousness within his body forced Xehanort to unlock his own heart yet again, the body fell into the Darkness. Aqua then sacrificed her Armor and Keyblade to save Terra/Xehanort. As Braig and Ansem the Wise helped an amnesiac Xehanort to care, Dilan carried Aqua's armor and Keyblade to Ansem's castle after the discovery of Xehanort, it was placed in the Chamber of Repose.

Kingdom Hearts II Final Mix
In Kingdom Hearts II Final Mix, Xemnas came before Aqua's Armor and Keyblade, addressing it as his "friend", which was noticed by Xigbar, who pointed this out to Zexion.

Terra's Armor and Keyblade (the Lingering Sentiment) appeared before Sora, Donald, and Goofy, mistaking Sora to be Aqua, then Ventus, then Riku, and finally Master Xehanort and attacking him in a great rage. Upon its defeat, the Lingering Sentiment realized it had attacked the wrong person, and respectfully acknowledged Sora as a Keyblade master.

Trivia

 * In the Birth by Sleep video, the Armor adorned by Terra, Aqua, and Ventus have capes. The capes were removed in Kingdom Hearts Birth by Sleep due to frame rate issues.
 * Nomura stated in an interview that he chose armor to hide Terra, Aqua and Ventus's faces so as to avoid using the black Organization XIII coats again; as well, the release of Kingdom Hearts II coincided with that of Final Fantasy XII, which also features armored beings.