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.

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 within her newly created 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, 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
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, making him a formidable opponent.

Kingdom Hearts 358/2 Days
In Kingdom Hearts 358/2 Days, Xion, dons armor which changes a number of times throughout her fight with Roxas. Her basic form is a bizarrely-shaped suit that is predominantly silver and black, with some dull red armor near her waist, gold on her boots, and orange armored gloves. Xion's helmet sports two black spikes that jut out horizontally on either side and a large, red Nobody symbol on the faceplate, part of which forms something of a horn on the top of the helmet.

In her first fightable form, Xion has large, skeletal, colorful wings and a huge, orange and red, intricately-shaped sword. In her second form, Xion loses her wings and grows to be about twice the size of Roxas, summoning a large, scythe-like weapon vaguely resembling the one of the Chimaera's arms as a weapon.

For the third section of the battle, Xion transforms into a figure with four huge arms, decorated with elegant, gold, purple and red designs. She also has large blades instead of hands, resembling Kurt Zisa's own arms. Her main body is also slightly larger than normal during this part of the battle. In her final transformation, Xion grows to titanic size, about the same height as the Train Station tower. Her armor is now embroidered with numerous spikes and ribbons. She also gains two Keyblade-like, pink and silver weapons with the teeth in the shape of the Nobody symbol as well as Sora's signature crown's spot is replaced with a Nobody symbol.

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; as well, the release of Kingdom Hearts II coincided with that of Final Fantasy XII, which also features armored beings as Judge Magisters.