As her closest friend and confidant, Flounder is incredibly faithful to Ariel, as well as protective. Part of his reasoning for joining her various exploits is to watch out for her should things go wrong. Though he would rather her hobbies were less perilous, Flounder will stick up to those who reprimand Ariel for her adventurous spirit, including King Triton at all times.
Flounder is essentially the polar opposite of Ariel. He is often riddled with apprehensiveness, frightens easily and hates adventure (for the most part). Like Sebastian and King Triton, Flounder is something of a "voice of reason" as he tries to talk Ariel out of adventuring for her own (as well as his) safety. Unfortunately, Flounder's agitation never get through. When he gets "cold fins", in fact, Ariel will (and effectively) pressure Flounder into tagging along from time to time. While he is not necessarily insecure or prone to peer pressure, Flounder dislikes to be called "a guppy" (a baby, in underwater speech), and will join Ariel on her forbidden exploits to prove himself. However, Flounder can be lion-hearted and pull through when Ariel really needs him. For example, during the last confrontation with Ursula in The Little Mermaid, Flounder jumped into the battle by taking on Flotsam and Jetsam.
The Little Mermaid
Flounder goes aside Ariel who is in the faraway outlands when trying to avoid Glut. He is later seen during Ariel's signature song, "Part of Your World". Later, the two go after Sebastian thinking why the human world is dangerous and must live under the sea. After that, Sebastian sings his signature song ("Under the Sea"). Flounder and Ariel feel sad after King Triton destroyed the statue of Prince Eric, meaning that humans and mermaids never get along. He and Ariel are later seen in Ursula's cave and during her song, "Poor Unfortunate Souls". As a human, Ariel meets Eric and then Flounder is seen on the marsh where Sebastian sings his second signature song, "Kiss the Girl". After that, Ursula sets a storm to attack Atlantica and Flounder is seen with the other aquatic animals during Ariel and Eric's wedding after the witch's death.
The Little Mermaid (TV series)
Flounder appears in all the episodes of the prequel television series, sharing constant adventures with Ariel. He generally tries to serve as a voice of reason in comparison to Ariel's more adventurous nature, but usually goes along with her anyway. He is often seen giving rides to Sebastian, who cannot swim as fast as Flounder or Ariel.
The Little Mermaid II: Return to the Sea
Flounder is first seen along with King Triton and Sebastian, attending the celebrations of the birth of Ariel and Eric's daughter Melody. He is disappointed when Ariel decides to keep all knowledge of the sea away from Melody in order to protect her from Ursula's sister Morgana. Following this, Ariel and Eric have a wall built around their kingdom to separate it from the sea, thus preventing Flounder from seeing Ariel again.
12 years later after Melody flees home and goes out to sea, Ariel is turned back into a mermaid by Triton in order to look for her. When returning to the sea, Ariel sees a fish that looks like Flounder and believes that it is him. Ariel goes to greet the fish, but scares him away. The fish returns moments later with his father, who turns out to be Flounder, who is happy to see Ariel again.
Flounder helps Ariel look for Melody. They see Morgana's pet rays Cloak and Dagger and follow them to Morgana's lair and are captured by her. Flounder is imprisoned with Melody, but they are freed by Melody's friends Tip and Dash.
After Morgana's defeat, Flounder is seen being happy that the wall separating the kingdom and the sea is destroyed by Melody.
The Little Mermaid: Ariel's Beginning
He has a larger role in The Little Mermaid: Ariel's Beginning, in which he first meets Ariel (contradicting the TV series) and later unknowingly leads her to the Catfish Club, where he works as a waiter, but dreams of being a part of the Catfish Club Band. His characterization is notably different in this movie; he doesn't scare as easily and is much more carefree and high-spirited. It is possible that he became more worrisome after the first few adventures with Ariel. He's obsessed with music, constantly humming and beatboxing even at inappropriate moments, which, given that music is forbidden in Atlantica, lands him into trouble on more than one occasion, but which also helps others keep their spirits up, most notably when his beatboxing kicks off the reprise of "Jump In The Line (Shake, Senora)" when he, Ariel, Sebastian, and the Catfish Club Band are on the run from Atlantica.
House of Mouse
Flounder made occasional cameos in the television series House of Mouse. A running gag in the show includes Flounder being served alive as Ariel's dinner. This was notably seen in "The Stolen Cartoons" and "Goofy for a Day".
In the episode "Suddenly Hades", Pete flooded the club to prevent Hades from attending the show. However, Ariel, Flounder, and Sebastian stayed as guests instead.
In "Big Bad Wolf Daddy", Flounder was seen amongst the crowd of Disney characters cheering on the Big Bad Wolf during his musical number.
Flounder makes a brief cameo in the Hercules (TV series) episode "Hercules and the Apollo Mission", seen after Hercules accidentally knocks Icarus into a fountain, who emerges from the water with Flounder.
In The Jungle Book 2, Flounder briefly appears as a freshwater fish when Bagheera and Colonel Hathi tumbled into the river.
In Moana, Flounder is seen amongst the tapa cloth fishes during "You're Welcome".
Flounder and Sebastian also starred in commercial bumpers for the Disney Junior along with an octopus, a carp, a snail, and a fluke who are Sebastian's musicians. These short bumpers showcase Flounder and Sebastian engaging in activities, accompanied by a song from an off-screen singer.
The Little Mermaid Musical
Flounder appears in the stage adaptation of The Little Mermaid. His supporting role is similar as in the film, but he does not give Ariel the statue of Eric and does not help Ariel reach Prince Eric's wedding barge, as the Vanessa subplot has been removed. However, Flounder performs a new song titled "She's in Love", which he sings with Ariel's sisters when they notice that Ariel has been acting "fishy lately". The stage role was originated by Cody Hanford and J.J. Singleton, but the two actors had to leave the show when their height overshot that of Sierra Boggess, who originated Ariel. The role was taken over by Trevor Braun and Brian D'Addario. On the Original Broadway Cast Recording, Brian D'Addario performs as Flounder.
Kingdom Hearts series
In the series of video games, Flounder appears as a relatively minor character in The Little Mermaid's based world, Atlantica. In the original game, as well as Kingdom Hearts II, Flounder takes the job of teaching Sora, Donald, and Goofy how to swim flexibly in the ocean waters after their arrival to the world
In Kingdom Hearts: Chain of Memories, however, Flounder's role is expanded fairly largely, as he's used as a toy in Ursula's quest for power, holding him hostage and forcing Ariel to choose between giving up the trident or Flounder losing his life. Ariel gives in, and hands over the weapon, but Sora and the gang are able to defeat her before it was too late, saving both the sea and Flounder.