This page contains unmarked spoilers. You Have Been Warned.
Fly
With his optimistic and brave attitude he is a likeable boy. Fly hates being stuck with his boring cousin, who never wants to join in the mischief. However, during the story, Fly learns to appreciate his cousin's abilities and realizes the necessity of thinking before acting, if only once in a while.
- Aerith and Bob: Chuck and Stella, though old-fashioned, are normal names. But Fly isn't a usual name at all. Granted, it could be a case of Only Known by Their Nickname.
- Big Brother Instinct: Towards Stella.
- Cool Board: His primary mode of transportation.
- Dub Name Change: In the Danish version, he's named Svip.
- The Japanese dub changes his name to Tobi, from "tobu", meaning "to fly". Similarly, the movie itself is retitled "Tobi Wars".
- "Eureka!" Moment: When Joe is escaping the flooded lab with the remaining antidote, Stella angrily calls him a "stupid fish". This gives Fly the idea to go after Joe and trick him into drinking more antidote so that the latter can turn into a human underwater and drown.Stella: Fly! What are you doing?
Fly: Using my brain! For once... - Fishing for Sole: He reels in an old can, which happens to contain Sasha.
- The Leader: Of the Headstrong type.
- Smarter Than You Look: He gets the idea to ask Joe many difficult questions to force him to drink more antidote turning him into a human underwater, drowning him.
Chuck
Chuck is Fly & Stella's cousin and is an introvert, a nerd and a loner, who would rather use his time to study genes and DNA molecules, than spending time with his peers.
Chuck dislikes visiting his irritating and mocking cousin Fly, with whom he has nothing in common. He is an only child and was brought up by his mother, Anna, in a stern and overprotective yet loving manner. He always does what mommy says he should and thinks before he acts and is therefore relatively devoid of initiative on all levels, except the one concerning expansion of his intellectual horizon.
Throughout the story, Chuck is forced to take responsibility and act on his own initiative to save himself and his two friends. That is the breaking point for Chuck personally, but also his friendship with Fly.
- Deadpan Snarker: Has tendencies of this.
- Dub Name Change: In the Danish version, he's named Poul, though everyone but his mom calls him "Plum" (probably derived from "plump" which means "chubby" in both Danish and English).
- In the Japanese version, he's called Jerry. This is a case of Bilingual Bonus as Jerry sounds similar to "Jelly" and he becomes a jellyfish.
- Heroic BSoD: When he believes Fly is dead he comes up with all crazy ideas to bring him back to life. Turns out Fly was alive all along.
- Hidden Badass: He takes out a collective of piranhas with martial arts.
- Irony: He's the smartest and most well-educated of the three protagonists, but gets transformed into a jellyfish, an animal that literally doesn't have a brain at all.
- Took a Level in Badass: Toward the end, he finally takes initiative to make himself and his cousins human again, not to mention singlehandedly taking out a tank full of piranhas.
- The Smart Guy: The one with the science knowledge of the trio.
- Super Drowning Skills: During the beginning of the film, he has this problem. After the three of them have been transformed, he explains to Fly that this was the reason he had no other choice but to drink the potion after MacKrill's boat capsized during the storm while they were looking for Stella. He gets better at it after becoming a jellyfish, however.Chuck: I can't swim…
Fly: Why didn't you say so before?!
Chuck: I had to drink the potion.
Stella
Fly's little sister is a happy little girl, straightforward and curious like most children her age. Stella is very close to her big brother, just like he is very protective of her. During the story, Stella is transformed into a little starfish and with the open mindedness of a child; she easily adjusts to the new circumstances in the underwater world.
- Annoying Younger Sibling: Totally averted here, as she and Fly appear to get along very well.
- Cheerful Child: She's very sweet and happy.
- Irony: She's a very good-hearted and cheery girl, but gets turned into a starfish, which are unintelligent, yet highly aggressive predators who even have cannibalistic tendencies.
- Meaningful Name: Her name is the Latin word for "star", and she winds up getting turned into a starfish.
- Tagalong Kid: The youngest of the trio, who is very close to her big brother.
Joe
Joe, until he was transformed, spent his life as a fish by cleaning the Shark's teeth. When the two are transformed, they exchange roles. Both of them do acquire intelligence, but Joe gets considerably more than the Shark. Joe realizes immediately that he now has the position of power in the relationship.
A personality blend of TV evangelist, serial killer and demagogue makes him with the Shark as his faithful underling into the villain of the film, driven by an insatiable thirst for power. He is calculating, egotistic, manipulative and intelligent. His character develops gradually throughout the film. At the start, he is charming and humorous, but the more he drinks he gradually becomes psychopathic and menacing.
- Body Horror: His transformation into a human as Fly tricks him into drinking more and more of the potion. His brain appears to be growing out of his head as his face grows more and more deformed with each swig, and at what would be his last drink of it, his scales slough off of his body to give way to flesh.
- Big Bad: He serves as the main antagonist of the film.
- Clothing Appendage: The markings on his body appear to be invoking the look of a suit and bowtie.
- Evil Brit: Helps that he's voiced by Alan Rickman.
- Hoist by His Own Petard: Fly tricks Joe into drinking more and more of the potion by asking him questions. As he evolves and his intelligence increases, he becomes more human before he eventually drowns to death.
- My Brain Is Big: Played for horror. In the climax, as he drinks more and more of the fish-to-human potion to make himself more intelligent, his brain outgrows his cranium and bursts out of his forehead.
- Terminal Transformation: Near the end of the movie, Fly tricks Joe into drinking more of the potion by asking him questions, causing him to become smarter and more human-like... until he loses his gills and drowns.
- Tom the Dark Lord: With a name like Joe...
Shark
Joe's henchman, the Shark is stupid but dangerous. Badly treated by Joe, he does not have the brains to rebel and simply continue to carry out his orders, occasionally feeling rather hard done by.
- A Dog Named "Dog": In contrast to Joe who claims a name of his own, Shark simply doesn't bother with anything than "Shark" for himself.
- The Dragon: To Joe, serving as his muscle and executioner for anyone that draws Joe's anger upon them.
- The Dog Bites Back: After having being crossed by Joe one too many times, he has enough with the little liar and confronts him head on, catching them both in a whirlpool in the process. Joe only escapes by a stroke of luck on his part and misfortune on Shark's.
- Dumb Muscle: Does not matter if he is plenty times bigger than his "boss", as long he gets to eat, he's satisfied with following orders.
Crab
Crab is the commander of Joe's army, he is a military-minded crustacean who takes his job as a soldier very seriously. unlike Shark who is very dimwitted, Crab is a cunning leader, and very brutal. As shown in the film, Crab is easily angered by Fly when he calls him wimpy. He was willing to betray Joe after he gets insulted by him for asking if he wanted to be his right hand man. When he drinks the potion after wounding Fly, he becomes more arrogant and egotistical, to the point of betraying Joe and stole his role as ruler of the Ocean.
- Big Bad Wannabe: He proclaims himself the new king of the ocean after knocking Fly down and drinking the last potion, but only briefly takes charge of the army of crabs before getting eaten by Shark due to his own smug attitude.
- The Dragon: One to Joe.
- The Starscream: After injuring Fly, he believes that drinking the potion will be the perfect opportunity to betray Joe and become the ruler of the ocean,after drinking the potion and mutating, he was very overjoyed of becoming the new ruler of the fish (before getting eaten by Shark afterwards).
- Not-So-Harmless Villain: He may seem like a harmless Dumb Muscle, but he manages to almost fatally wound Fly with just one punch.
Professor Mackrill
The professor is, as most highly intelligent people, a bit confused and absent-minded. He has lived most of his life secluded from the rest of the world and has therefore forgotten how to socialize or even how to have an ordinary conversation. Apart from being enthusiastic and devoted to his work, he is a really nice guy. He wants to like people but he doesn't understand them. This makes him lonely at times, but whenever somebody shows genuine interest in his work, he gets all fired up and can talk forever.
- Absent-Minded Professor: He shows many shades of this, but is brilliant, nonetheless.
Sasha
Plenty of little girls yearn for the companionship of an equine being and eventually earn their wishes. It is the case for Stella and the unbreakable bond she maintains with Sasha, an adorable land-converted seahorse. Seeing how she aided the three children into avoiding the mutated armies of Joe, in order to retrieve the antidote during their underwater journey, she is considered more as a close friend than a mere pet.
- Black Bead Eyes: Unlike most of the characters, be it fish or human, Sasha has black dots for eyes.
- The Speechless: As she didn't drink the antidote that makes fish talk, she's the only fish character in the film that doesn't speak.
Lisa and Bill
Fly and Stella's parents. They simply went out for the night, leaving their children in the care of their Aunt Anna while they were away. Coming home, they did not expect to find their kids missing, and they go in search for them.
Anna
Chuck's very doting and protective mother.
- Faint in Shock: Her response to seeing the video recording of Stella transforming into a starfish.
