I could tell you all about what a kappa is, but suffice to say that "Kappa the Imp" is a redundant phrase. Kappa isn't a personal name but rather a name for an entire species, so when "Kappa" shows up to explain Gau's rages to you, it's intended to mean "a kappa," not "this imp guy named Kappa." However, every other instance of the word "kappa" in the game was changed to "imp" so American players wouldn't have to worry about what a kappa was; unfortunately, leaving that one instance unaltered gave them the idea that "Kappa" was a unique name. "Kappa" and "imp" both refer to the same type of creature, so "Kappa the Imp" makes as much sense as "Feline the Cat."