Shortly after Sora meets Donald and Goofy for the first time in Traverse Town, a large horde of Heartless began to attack them. One of them was a Guard Armor, the leader of that horde. The trio fought the mighty Emblem Heartless, and eventually managed to defeat it. With the defeat of the Guard Armor, the other Heartless temporarily calmed down.
Despite this, the Guard Armor later appeared after Sora rung the Second District bell three times (though it's unknown if it's the same Guard Armor that Sora defeated earlier, or if it's a completely different one). By ringing the bell three times, Sora ended up discovering Traverse Town's Keyhole.
Just when he was about to seal the Keyhole, a Guard Armor showed up out of nowhere and attacked Sora and his friends. After dealing enough damage to the Guard Armor, it appeared as if it was defeated.
However, the Guard Armor got right back up, and began to transform into another Emblem Heartless; Opposite Armor. Despite being much stronger, the Emblem Heartless ended up getting destroyed by Sora and his friends.
Kingdom Hearts: Chain of Memories
While Sora was exploring his memories, while he was in Castle Oblivion, he ended up in Traverse Town. He eventually came across Cid, to which Cid warned Sora about a monster that appears when the Second District bell begins to ring. Shortly after, the bell began to ring, and a Guard Armor appeared in front of Sora and proceeded to attack him. In the end, the Guard Armor ended up being defeated by Sora.
While Riku was exploring Castle Obilvion, he eventually came across his version of Traverse Town. While exploring, a Guard Armor showed up out of nowhere and attacked Riku. However, the Guard Armor was quickly defeated by Riku. It was eventually revealed that the Guard Armor was a manifestation of the darkness inside Riku's heart.
Kingdom Hearts 358/2 Days
While Roxas and Xigbar were fighting each other during a tournament in the Olympus Colosseum, their fight was interrupted by a Guard Armor that appears out of nowhere. Xigbar decides to step out and let Roxas take care of the Guard Armor on his own. In the end, Roxas was able to beat the Emblem Heartless and "win" the tournament.
Kingdom Hearts coded
After Data-Sora managed to unlock the Keyhole of the Data version of Traverse Town, he eventually encountered a data version of the Guard Armor. Sora thought that the Guard Armor was the source of the Bug Blox that had been corrupting Data-Traverse Town. Data-Sora confronted the Guard Armor and ended up defeating it, causing Data-Traverse Town to be restored.