The Kingdom Key (キングダムチェーン Kingudamu Chēn?, lit. "Kingdom Chain") is the most recognizable Keyblade in the Kingdom Hearts series, as it appears within each game of the series, and frequently serves as the default Keychain for cutscenes. In Kingdom Hearts: Chain of Memories, it is the only Keychain that Sora can visibly use, while in Kingdom Hearts 358/2 Days, it is the only Keychain of Xion's to appear in Story Mode, and excluding the final Mission, the only Keychain of Roxas's to appear in any cutscene. Furthermore, while Kingdom Hearts II normally depicts the equipped Keyblade within cutscenes, the Kingdom Key appears by default in the cutscene leading up to the Battle of 1000 Heartless, and within the Album pictures of Jiminy's Journal. In Kingdom Hearts 358/2 Days, it can be upgraded to the Kingdom Key+ and the Kingdom Key++.
The Kingdom Key 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. The Kingdom Key is not wielded by Sora exclusively; Riku, Roxas, and Xion have also used it, and King Mickey is often seen wielding the Kingdom Key D, a color-inverted version of the weapon.
For Roxas and Xion, the Keyblade they wield is in fact Sora's Keyblade, existing in multiple places at once. Roxas also obtains a second Kingdom Key after absorbing Xion; however, this is in fact Ventus's Keyblade, not Xion's.
The Kingdom Key D, a color-inverted Kingdom Key which King Mickey wields, is the Keyblade from the Realm of Darkness, which he obtained to mirror Sora's Keyblade from the Realm of Light.