Following is a list of the important characters in Help! I'm a Fish. Voice actors are listed from the English version only.

Fly

Arguably the lead protagonist and main character of the film, Fly is voiced by Jeff Pace. (I say arguably bcause while Chuck falls into the typical role of the nerdy overweight sidekick, Chuck takes on a strong hero role especially during the second half of the story.) When he takes the fish potion Fly is turned into a California flying fish (Cypselurus californicus). In the film they persist in calling it a "California flyfish" which is a sometimes-used name, but most notably the fish Fly becomes looks nothing at all like a Californian flying fish. Oh well. As for his name, it's likely enough that "Fly" is not his true name but his nickname, earned for the fact that he loves fishing.

Joe

The main antagonist of the film, voiced by Alan Rickman. Joe is apparently a pilot fish (Naucrates ductor), despite the lack of stripes. At the start of the film he is in "service" to the Shark but upon receiving a dose of antidote turns into a power-hungry dictator bent on conquering all life in the ocean.

Chuck

Chuck (or Charles) is Fly and Stella's first cousin, and voiced by Alessandro Juliani. When he takes the fish potion he becomes a jellyfish, possibly a moon jellyfish (Aurelia aurita). He's as smart as a computer and rather snooty at first, not to mention extremely pessimistic. The events of the film probably serve to change him for the better more than any other character.

Stella

Fly's little sister, voiced by Michelle Westerson. The fish potion turns her into a starfish - if I had to classify her I'd say the common cushion sea star (Oreaster reticulatus) as it is the right shape and can be yellow/orange. She's young, innocent, and typically adorably precocious.

Sasha

Sasha is a short-snouted seahorse (Hippocampus hippocampus or the synonym, Hippocampus vulgaris), voiced by Louise Fribo. She never takes the antidote and her intelligence level is supposedly normal for a seahorse (well, it is a cartoon). Stella tames her and at the very end of the film the Professor and Chuck come up with a formula that transform her into a terrestrial horse, although still green and yellow and without external ears.

The Shark

The Shark (who was never named) is probably a great white shark (Carcharodon carcharias) and is voiced by David Bateson. He didn't become as intelligent as most of the other fish by taking the antidote, possibly due to his large mass (its as good an explanation as any). He inexplicably has a sort of Cockney-esque accent; but apparently it is possible for a great white to travel large distances so who knows, maybe when fish gain the ability to speak via the antidote they also get the accent of the area in which they were born? Again, its as good an explaination as any...

Professor MacKrill

The marine biologist Professor H. O. MacKrill graduated from the University of Copenhagen and is voiced by Terry Jones. He hypothosises that eventually humans will cause such global warming that the polar ice caps will melt and flood the Earth, so he's invented "Fish Potion" which will enable humans to turn themselves into fish and thus continue to inhabit the planet. Having the foresight to come up with an antidote was just a bonus.

The Crab

The Crab was the first creature Joe deliberately used the antidote on, and is voiced by David Bateson. I'm not sure what type of crab he's supposed to be; I don't think he's a king, perhaps a stone crab. He is a moronic military soldier with a desire to move up in the ranks.

Anna

Anna is Chuck's mother and Fry and Stella's aunt, and voiced by Pauline Newstone. I'm not sure which of Fly's parents she is related to, unless it is/was her husand who's a sibling. If she has a husband he is never mentioned or seen, and if he's out of the picture we don't know if he's deceased or if it was a divorce. Anna is your typical bossy pushy large woman you always see in movies; but you have to give her credit for her close bond with her son. And I mean close - more than once in the film she has a sort of psychic connection with Chuck, somehow knowing when something dramatic has happened to him.

Bill

Fly and Stella's father.

Lisa

Fly and Stella's mother.