After the death of Queen Athena, the distraught King Triton appointed Marina Del Rey as the governess to his seven young daughters. Under Marina, the girls follow a strict schedule. However, unknown to the royal family, Marina secretly loathed her job. Instead, she dreamed of being King Triton's attache, a job held by Sebastian.
But that never meant she never cared for the girls though. It is seen in the early part of the film that she does feel some sort of sympathy for them and their harsh life, when seeing the sad faces on the girls when King Triton dismissed Ariel's suggestion to go swim at the reef, instead of the morning chores. Marina expresses a small sad face, possibly one of sympathy, towards the girls and their wish for a more fun and happy life. It is also shown that during the castle walk, when Ariel attempted to play with one of her sisters, Marina tried to quiet them down. It could have been for a number of reasons: her job as Governess, wanting to prevent the girls from getting in trouble, or trying to keep herself out of trouble for not controlling them.
Marina's dream of getting Sebastian's job did not become an obsession until the crab tells the king that he is considering changing the castle staff. This caused her to snap slightly; those simple words, along with him constantly berating and degrading her, as well as always pointing out her flaws (such as being late that one morning), made her worried about losing her job. Soon she decides that she is willing to do anything to prevent her self from losing her cushy job at the palace, as well as gain the crab's job.
One day, Marina discovers that Ariel and her sisters have been sneaking out to an underground music club, which is led by Sebastian. As music is banned in Atlantica after Athena's death, Marina reports the news to King Triton. Triton imprisons Sebastian, as well as the other music club members, and confines his daughters to the palace. As a result after Ariel confronts her father about her mother not banning music, Marina rises up and assumes Sebastian's position.
In an attempt to make Triton remember how much he used to love music, Ariel and Sebastian retrieve a music box, which he had given to Athena as a gift before she died. On their return, Marina confronts them. Before music is restored back to Atlantica, the final battle begins when Marina bans Ariel from heading back home. Marina wishes to retain her new position, and attacks Sebastian. Ariel pushes her away, but is injured in the process. Triton arrives in time to witness this, and regrets his earlier actions.
The sea king decides to end his musical ban and restores music back to Atlantica, and next appoints Sebastian to the position of court composer. Music is allowed in Atlantica, and Marina is imprisoned by Triton, thanks to Ariel.
- Marina Del Rey's mental breakdown and her desire to keep her job by any means necessary was caused by Sebastian's constant berating and degrading. So, by a technicality, Sebastian could be seen as a secondary antagonist: if he had not constantly mistreated Marina, chances are she never would have snapped in the first place.
- When she fights with Sebastian, it resembles the fights between Prudence and The Grand Duke to impress The King in Cinderella II: Dreams Come True
- When compared to her predecessors, Marina is only a villain in the loosest sense of the word (all she wanted was a promotion, where as the villains before her wanted the whole kingdom under their thumb). Unlike Ursula and Morgana, Marina is a mermaid (just like the film's protagonist) and is a staff of the palace.
- Marina is the second main villain in the franchise to have Eels as henchmen (the first being Ursula). However, unlike Flotsam and Jetsam, her eels don't talk and they're Electric Eels instead of Moray Eels. Also, Marina has six eels as opposed to Ursula's two.
- It is shown that when ever Marina changes her clothes, her fins and tail also change color as well (seen here, here, here, and here). This could be either an animation error, or it is possible that mermaids can change the colors of their fins and tails in some unknown way.
- She also could be putting fabric on her tail and fins, or because of the changing colors of lights.
- Marina has some similarities to Yzma. Aside from having some physical similarities to her, she also had a similar motive towards her villainy: Both were the result of wrongs committed against them by one of the main characters (Sebastian's berating and degrading of Marina, and Kuzco's callously firing Yzma, despite loyal servitude to Kuzco's dynasty for several years).
- Marina Del Rey is the only The Little Mermaid villain to be a merperson, as all prior villains were Octopids (Ursula and Morgana), sentient fish and crustaceans, the Moray, and Manta-like creatures (like Evil Manta).
- Her name coincides with Marina del Rey, a city in California.
- There was a popular theory by the fans which suggested that Marina and Ursula were the same person because of the similarities between the two characters. This was disproven however when the film was released on DVD.