Keychain
The Keychains are a somewhat special piece of equipment that upgrades a Keyblade, as seen with Sora's Kingdom Key. While the Kingdom Key appears to be the most basic form of a Keyblade, the Keychains actually increase its overall status', giving it a new appearance as well as granting special abilities. The power of enhancement in the Keychain may be caused by the relationship between one another, as evident when Riku reclaims the Kingdom Key in Hollow Bastion, despite having the Key, he cannot change or enhance the Kingdom Key. Furthermore, it might suggest that the stronger one's friendship with one another, the stronger the bond it will create, increasing the power of enhancement.
It should be noted that when Sora is equipping another Keyblade, he is actually re-equipping the Keychain, thus enhancing the Kingdom Key and changes its appearance. While Mickey's Keyblade and Riku's Way to the Dawn, (this weapon actually has a heartless emblem for a keychain), hold Keychains, some Keyblades lack a Keychain, as evident by Riku's Dark Keyblade forged from the Seven Princesses' Hearts. A theory suggests that while the Keyblade served as a "body" and its wielder serves as the "soul", the Keychain serves as a "Heart", since Xehanort's Heartless possessed Riku, he cannot gain access to the Keychain due to the lack of Heart.
