Keyblade Armor

Keyblade Armor is a metallic bodysuit worn by Keyblade wielders Terra, Ventus, and Aqua in Kingdom Hearts Birth by Sleep, 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. Despite its heavy appearance, the armor is actually quite flexible, allowing the same free range of movement that the user's normal clothes do.

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. Their armor is also seen whenever they go to the Mirage Arena, and aspects of it are also visible in their normal clothing; all three wear armored boots, and Ventus appears to wear armor under his jacket.

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 incarnation'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 Chamber of Waking within her newly reformed world of 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, and 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. It was then placed in the Chamber of Repose.

Kingdom Hearts 358/2 Days
When she is forced to fight Roxas to the death, Xion dons a suit of armor that combines motifs from the Keyblade Armor, the Nobody emblem, and Sora's own clothes. At times, the armor also has wings, extra arms, or trailing ribbons that draw from Sora's memories, and a set of "Keyblades" which do the same.

Kingdom Hearts II
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.

Xemnas dons what appears to be Keyblade Armor while attacking Sora and Riku atop his own Dragon Form. During this fight, Xemnas is also able to use the weapons of the recently destroyed members of Organization XIII.

Trivia

 * In the Birth by Sleep video, the Armor worn 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, although the coat would later be used to obscure the identity of the Unknown in the game's English and Final Mix versions.
 * While Xemnas himself never wields a Keyblade, his apparent use of Keyblade Armor appears to hint that he could use a Keyblade, but just preferred not to.