After thinking about the name and the secret ending, I think I now know what the Chasers are. I believe, The Chasers are a group of individuels who seek the power of the keyblade. I also believe they wield false keyblades that lack the real keyblade's power but the Chasers want to drain the real keyblades power to make their falseblades all powerful( and that would make The Chasers all powerful as well).