The various pieces of the Opposite Armor, known as "Armored Torso", "Gauntlets", and "Hammerlegs", are able to move independently.
 Kingdom Hearts
After sealing the Keyhole in Deep Jungle, Sora returns to Traverse Town and tests the legend about the Second District bell by ringing it three times, revealing the world's Keyhole within the district's fountain. However, as the group goes to seal it they are attacked by a Guard Armor, and though they defeat it, it transforms itself into the stronger Opposite Armor and attacks again. After it is destroyed, they are able to seal Traverse Town's Keyhole.
A second Opposite Armor appears in the Hades Cup.
The Opposite Armor's body is made of identical components to the Guard Armor, the key difference between the two being that the Opposite Armor's are inverted. Thus, its torso is upside down, with the collar-like additive now being on its underside. The Heartless emblem is emblazoned on the center of the armor. Its head is the only part of its body that is not actually inverted, but instead its faceplate drops down. This exposes that the Heartless does in fact share the pitch-black face and glowing yellow eyes of other Heartless. Both of the Gauntlets now act as the Opposite Armor’s legs, and it walks on its hands. The Hammerlegs take the places of its arms, and its feet replace its armored hands. Curiously, the Opposite Armor's Heartless emblem is right side-up, despite it being the Guard Armor's torso flipped upside-down. However, viewing its model in Jiminy's Journal reveals it to have two heartless emblems, one on the front, and an up-side-down one on its back.
In Kingdom Hearts Final Mix, the violet sections of the Opposite Armor are silver, its torso gains red, yellow, and blue stripes on its upper half, its head becomes red with yellow spikes, and the additives on its top and upper rims become blue and yellow, respectively. Its fingers become grey, while its feet become red.
The Opposite Armor's name references its armored body, as well as the fact that its body parts are the inverted pieces of the Guard Armor's body.
This Heartless is an inverted but stronger version of the Guard Armor. Like its predecessor, each part attacks individually or in concert. It also has more mobility due to the fact that it can levitate. It also has a powerful desperation move where it rearranges its body to give it the appearance of a cannon and shoot energy blasts from the torso.
To begin with, this battle will be reminiscent of your first boss battle in Traverse Town with Guard Armor, mostly because it will be exactly the same. However, once its health bar reaches a certain point, the body parts will completely collapse and rearrange themselves to form an even nastier opponent. Sora can leave the arena at will during the boss battle, though when he returns, the Opposite Armor will be at full health.
Opposite Armor uses all of Guard Armor's tricks, so you should be familiar with the spinning and stomping attacks. Avoid them, and attack during the cooldown period. Begin by taking out the Gauntlets, as they have the least HP and by doing so, will reduce the amount of stress considerably, followed by the Hammerlegs. Once all that remains is the torso, it will frequently execute a bazooka-esque attack. Avoid this using Dodge Roll and quickly perform an aerial combo. The torso will then attempt to escape, so make sure you follow it, or avoiding the attack will be difficult. Keep avoiding and attacking, and your enemy will eventually fall.
After you defeat the Opposite Armor, you receive a new spell, Aero, and another Navi-G Piece.
Note that you can enter and exit freely from the Second District during this battle, but if you have already taken out some of its body parts when you leave, it will have full HP and all its body parts with it when you re-enter into the Second District. Also, remember that the Opposite Armor is an airborne enemy, unlike the Guard Armor, so if you happen to climb up the upper level of the Gizmo Shop (just outside the bell tower), expect the Opposite Armor to hover above you.
 Notes and references