we've all noticed that the heart unlocker has no keychain. The heart unlocker is artificial and the only one of its kind that has the power to unlock hearts.
The Soul Eater was just a sword in the beginning. It didn't develop into a keyblade until KH2, as for the how and why can only be guessed. Riku was supposed to be the original Keyblade Master, whatever powers he holds its most likely that he himself was able to craft the Souleater into a keyblade of its own, not too mention that he was able to duplicate a keyblade for Kairi momentarily. I believe that the powers of Riku may indeed be somewhat coming towards the status of the title Master, just that no one knows entirely yet