Kingdom Key

The Kingdom Key (キングダムチェーン) is the most recognizable Keyblade in the Kingdom Hearts series, and is the form used in almost all depictions of the weapon. It has appeared in every entry of the series to date. The Kingdom Key also draws out the true power of Sora's Keyblade, even though it is not the strongest keychain. As such, it is the true form of his Keyblade, which means that it is also the default form of Roxas's and Xion's Keyblades. In Kingdom Hearts 358/2 Days, it can be upgraded to the Kingdom Key+ by attaching the Zero Gear (5).

The Kingdom Key first appears when Sora receives the Keyblade during the destruction of the Destiny Islands, in Kingdom Hearts, but is not wielded by Sora alone; Riku, Roxas, and Xion have also used it, and King Mickey is often seen wielding a color-inverted version of the weapon. For Riku, the Keyblade takes the form of the Kingdom Key when he takes it from Sora, and for Roxas and Xion, the Keyblade they wield is in fact Sora's Keyblade, existing in multiple places at once. The Kingdom Key also appears in the secret video The Gathering, where Terra pulls it from the ground in Sunset Horizons.

The Kingdom Key D, a color-inverted Kingdom Key which King Mickey wields, is in fact the Keyblade from the Realm of Darkness, which he obtained to mirror Sora's Keyblade from the Realm of Light. This purpose is reflected in its coloration - where Sora's Keyblade is silver, Mickey's is gold, and the same goes for their hilts. Even the rainguard, blue on Sora's Kingdom Key, is red on Mickey's.

Appearance
The Kingdom Key is depicted as a classic skeleton key approximately 3.5' long, with a long silver keychain extending from the hilt, and a Hidden Mickey token on the end of the keychain. The blade and teeth of the Keyblade are silver, the rainguard is blue, and the hilt is gold. The teeth of the Keyblade form the outline of a crown which matches the crown charm Sora wears.

Trivia

 * The Kingdom Key is the form depicted in all pre-rendered videos, as it is the only keychain which is possessed throughout each game. For example, no matter what keychain you use to fight World of Chaos, the cutscenes afterward depict the Kingdom Key in its place.
 * The use of a Hidden Mickey as the Keyblade's token may be due to Mickey being the main character in early plans for Kingdom Hearts.
 * In Sunset Horizons, Terra takes this Keyblade.