keychains are small mementos of someone's heart given to sora. when a keychain is attached in place of the Kingdom Keychain, it changes the Keyblade's appearance to represent that person's "heart." For example, crab claw and mysterious abyss represent Ariel's heart, the Gullwing the FFX-2 girls, etc. The greatest examples of this are Oblivion, which represents Riku, and Oathkeeper, which represents Kairi. There are certain keyblades that are born within the hearts of different realms. Mickey's is the Keyblade of Darkness, forged from Kingdom Hearts in the realm of darkness, while Sora's Kingdom Key was forged in the realm of light. See what I'm saying???