Ursula is the main villain of the 1989 film. She is voiced by Pat Carroll in the film, the spin-off television series and the Kingdom Hearts video games. She is the older sister of Morgana. Ursula is based on the "sea witch/sorceress" character in Hans Christian Andersen's story "The Little Mermaid". In the original story the sea witch is a neutral enabler, but for Disney's animated adaptation, the character was modified into a full-fledged antagonist and plays a larger role in the overall story. Ursula is a part-octopus sea witch who "helps" unfortunate merfolk to achieve her own goals. Ursula frequently turns them into polyps and imprisons them. Her appearance is of an obese purple-skinned, white-haired female human with a facial mole, but from the waist down she has six black tentacles. Her overall design is based on the drag queen Divine.

When Ursula first appears in the film, she states through monologue that she once lived in the royal palace of King Triton. She was banished from Atlantica due to previously trying to attack and seize the throne, and made her home in the remains of a leviathan. Ursula commands her minions, the twin moray eels Flotsam and Jetsam, to watch Triton's youngest daughter Ariel, who might be "the key to Triton's undoing". Ursula forms her plan to seize power around Ariel's forbidden love for the human Prince Eric. Flotsam and Jetsam persuade Ariel to go to Ursula's lair, claiming that the sea witch is the only one who can solve her problem. Via the song "Poor Unfortunate Souls", Ursula proposes a deal in which she will turn Ariel into a human for three days. If Ariel receives the "kiss of true love" from Prince Eric before sunset on the third day, her transformation will be permanent. However, if Ariel fails, she will turn back into a mermaid and be bound to Ursula forever. The price of transformation is Ariel's voice. Ariel signs a magical contract and Ursula takes her voice, and then transforms Ariel into a mute human.

When Ariel comes "too close" to succeeding, an enraged Ursula decides to sabotage the bargain and takes the form of a beautiful human woman named "Vanessa". Ursula uses Ariel's voice to hypnotize Eric into marrying Vanessa at dusk on the third day. Shortly before the wedding, Scuttle discovers that "Vanessa" is really Ursula in disguise and hurries to tell Ariel, and then helps to ruin the wedding. Complete mayhem ensues, and Scuttle destroys Ursula's necklace, restoring Ariel's voice to its original owner. Ursula's control over Eric breaks, and he reunites with Ariel. However, the sun sets just before Ariel and Eric can kiss, and Ariel reverts back to her mermaid form. Ursula, transforming back into her original form, claims Ariel and drags her back into the sea. Ursula is quickly confronted by Triton who demands that she release Ariel, but the tables turn when Ursula reveals that the contract Ariel signed is unbreakable. Ursula offers to let Triton take Ariel's place in the contract, setting Ariel free, but in exchange Triton must forfeit his power. Triton agrees and is transformed into a polyp. Ursula takes the crown and magical trident, claiming herself as the new ruler of the seas. Meanwhile, Prince Eric decides to go rescue Ariel and fight Ursula. A small battle ensues undersea, culminating with Ursula accidentally killing Flotsam and Jetsam with the trident. After mourning for her pet eels, an enraged Ursula grows to titanic proportions and begins creating a massive oceanic storm and whirlpool, which brings up several submerged ships from the depths. Eric manages to board one of these ships. In order to ensure that there are no other heirs to the throne, Ursula traps Ariel inside a whirlpool she created. Ursula then tries to execute the mermaid princess with blasts from the trident. Ariel just barely manages to survive each blast. Ursula then raises the trident and prepares an unavoidable, fatal blast for Ariel. The princess gapes in horror, certain of her doom. Just before Ariel's execution can be carried out, Ursula is defeated by Eric, who drives the sunken ship's broken mast into her abdomen. With Ursula gone, her evil spells also cease to function; the polyps turn back into merpeople and escape from Ursula's hideout.

Ursula appears as a occasional villain in four episodes of The Little Mermaid prequel television series: "Against the Tide", "Tail of Two Crabs", "Heroes" and "Ariel's Treasures". In all four episodes, she executes various plans to antagonize King Triton and take over Atlantica, but all fail. In the first two episodes, interaction between Ariel and Ursula is kept at a minimum, but in the later ones, Ariel and Ursula face each other more directly. Ursula does not appear in the direct-to-DVD film The Little Mermaid II: Return to the Sea, but she is mentioned many times, mostly by her sister Morgana and is seen as a picture with a nice face. In the family portrait during the cut song "Gonna Get My Wish", Ursula was also depicted with teal skin similar to Morgana.

Ursula appears in Disney park attractions such as the Fantasmic! show debuted in 1992 at Disneyland Park, as one of the Disney Villains summoned to destroy Mickey Mouse. Ursula is a central character in the annual Halloween-themed fireworks show HalloWishes at the Walt Disney World Magic Kingdom Mickey's Not-So-Scary Halloween Party "hard ticket" event. Ursula also appears at the tail end of the Little Mermaid unit in Walt Disney's Parade of Dreams at Disneyland.

In the Kingdom Hearts video game, Ursula appears as one of Maleficent's co-conspirators, using the power of the Heartless to attack Atlantica and gain power. In this version of events the official walkthrough states that Ursula was King Triton's fortuneteller before she was banished. Ursula is eventually defeated by Sora, Ariel, Donald Duck and Goofy. In Kingdom Hearts: Chain of Memories, she appears as a facsimile created from Sora's memories. In Kingdom Hearts II, Ursula mysteriously returns through the powers of darkness and appears to Ariel, and recreates her film role. As the film's storyline is retold in the game, Ursula does not recognize Sora and co., and Ariel makes no mention of her defeat in the original Kingdom Hearts. In fact, when Ursula arrives to make the deal with Ariel, the latter reacts as though she has seen Ursula for the first time. Sora, however, does note that Ursula "got what she deserved" in the first game. At the story's climax, she is defeated when Eric hurls the trident straight through Ursula's chest, destroying her seemingly once and for all.

In the musical version of the original film, Sherie Rene Scott originated the live role, which she played until January 25, 2009. Heidi Blickenstaff played the role from January 27, 2009 to April 2009. Faith Prince took over the role April 7, 2009. In this version, Ursula is King Triton's sister (an original concept for the original film that was eventually dropped). As revealed in the lyrics of Ursula's song "I Want the Good Times Back", when their father died, the pair were given equal share of the sea plus two magical items. Triton received the trident while Ursula received the magic Nautilus shell. Though the two were to rule the seas together, Ursula's greed and use of dark magic to usurp Triton led to her being banished. The need for revenge is her motivation for the show.

In the first two issues of the official "The Little Mermaid" comic, Ursula appears briefly to negotiate with a race of eel-men for the carcass of a Leviathan which she would convert into the home seen in the 1989 film.

In the Disney Press book "The Villain Files", it is implied that Ursula is Ariel's aunt, a concept that is based on the abandoned story concept of the 1989 film The Little Mermaid. However in the book My Side of the Story: Ursula she says "In rare moments of quiet, my mind conjures up the same image: a handsome older man with fabulous hair, wearing a crown and carrying a trident. His job is pressured, and I can see that he's stressed. If anyone needs a healing massage, it's King Triton. When he books an appointment, I'll be waiting." So then with this said she cannot be Triton's sister who was removed from Atlantica for usurping the throne.

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