Joe is voiced by Alan Rickman.
Joe's main ambition is to gain more of the antidote to become more intelligent, create a huge army, and to take over both land and sea.
In the final battle, the film's hero, Fly, manages to trick Joe into drinking much more of the antidote, by challenging his intelligence. Joe gradually transforms into a mutant fish-like human, and Fly asks him a final question: can a human breath underwater? Joe answers "Of course not!" and realizes it was too late, as he drowned. His lifeless mutated body is then carried away by the current.
Joe is a pilot fish that is described as being "inspiring" to other fish. He holds the power of persuasion, and he uses that to his advantage in recruiting others to join him. The only one who resists his offers of power is Fly. In the English dub, his smooth voice was given by Alan Rickman.