Keyblade Armor

Keyblade Armor (キーブレード鎧) is a suit of armor worn by Keyblade wielders. It first appears in Kingdom Hearts II Final Mix, as the remnants of Terra and Aqua, and is used throughout Kingdom Hearts Birth by Sleep by Terra, Ventus, Aqua, and the remnants of Master Eraqus and Master Xehanort. One of its most important qualities is that it protects the wearer from the darkness surrounding the Lanes Between, much like the Black Coat. However, it is also worn during battle, at the user's discretion.

Pieces of the armor usually appear as part of the user's normal clothing, such as their boots or belts. However, the armor's left pauldron is more important—by pressing it, the user activates the rest of the armor, which appears in a flash of light. Despite the fact that it usually appears and disappears magically, the armor can also be removed piece by piece, as Terra-Xehanort did once he had gained control of Terra's body, and Aqua did to give Terra's body a vessel to take him out of the Realm of Darkness. Despite its heavy appearance, the armor does not appear to impede movement in any way.

Kingdom Hearts Birth by Sleep
According to his Reports, Master Xehanort had once worn his Armor as instructed, but when he felt the darkness of the Ocean Between, he decided to embrace it, rather than avoiding it.

Terra, Ventus, and Aqua wear their Armor whenever they enter the Lanes Between, or when they land at the Mirage Arena. They also activate the armor when they are attacked by Xehanort at the Keyblade Graveyard.

At the Mirage Arena, Terra, Ventus, and Aqua can change the hues of their armor.

When Terra defeats Master Xehanort in their final battle, the elderly Keyblade master unlocks his own heart. Terra attempts to protect himself from Xehanort by quickly activating his armor, but Terra's darkness allows Xehanort to take control anyway. Terra's armor falls to the ground in pieces as the new, younger Terra-Xehanort walks off, but rises again as Terra's soul animates it. It creates the Will's Cage around the two, forcing Terra-Xehanort to turn and fight. It manages to knock out Terra-Xehanort, who is swallowed by the χ-blade's aura and transported to Radiant Garden. Its job done, it kneels down with its Ends of the Earth Keyblade in its hands, and remains there for the next eleven years.

After leaving Ventus in the Chamber of Waking and encountering a confused Terra-Xehanort at Radiant Garden, Aqua damages Terra-Xehanort enough for Terra's heart to temporarily take control and lock away both his and Xehanort's hearts, knocking his body out. Both Aqua and Xehanort fall into the Realm of Darkness. Aqua removes her armor, and instructs both it and her Keyblade to take Xehanort back through the closing portal to Radiant Garden. Dilan later discovers the unconscious Xehanort and Aqua's armor, and Ansem instructs Braig to carry Xehanort, and Dilan to bring the armor back to the Hollow Bastion.

After Xehanort became Xemnas, he placed Aqua's armor within the Chamber of Repose, where he often went to speak to it.

Kingdom Hearts 358/2 Days
When she fights Roxas, 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
A portal to the Keyblade Graveyard appears at Disney Castle at one point, and Sora, Donald, and Goofy enter it to find Terra's Lingering Sentiment. It first mistakes Sora to be Aqua, then Ventus, then Riku, and finally Master Xehanort, before attacking him in a great rage. Upon its defeat, the Lingering Sentiment realizes it has attacked the wrong person, and respectfully acknowledges Sora as a Keyblade Master.

Xemnas dons Xehanort's Keyblade Armor while attacking Sora and Riku atop the World of Nothingness. 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; furthermore, he was influenced by the recent release of Final Fantasy XII, which featured the armored Archadian Judge Magisters.