Princess Marisa is the daughter of King Pescoro and Queen Camila of the Sirenas, the merpeople of Avalor that live in Nueva Vista, making her Crown Princess of her people alongside her brother, Crown Prince Marzel. Their only appearance so far was in the one hour special, "Song of the Sirenas".
Due to her mother, Queen Camila, having originally been human before King Pescoro turned her into a Sirena to save her life from drowning, Marisa is half-human, and half-Sirena. As a result, whenever she is on land, her Sirena tail turns into a pair of human legs, allowing her to walk on land. Her brother Marzel also gained this ability. However, the first time Marisa tried this, she was mistaken by a pair of fishermen for being a human baby, and they tried to take her away from Pescoro and Camila. Though her parents got her back and escaped, Pescoro was given a deep fear of humans after that incident, and wanted his people to stay away from them, even though they no longer sank ships after Pescoro saved Camila.
Marisa, Marzel, and their seahorse buddy Coco, are on the surface of the ocean when they are seen by the Royal Yacht carrying the Avalor Royal Family. Upon seeing them, Naomi Turner's parents prepare to open fire on them with their crossbows, only to get off a few shots before Crown Princess Elena intervenes, confronting them over attacking the Sirena Crown Prince and Princess when they had not shown any ill intent towards them first. As a result, Marisa and Marzel return to their parents to report on what happened, but having found out they were out and about at the surface, Pescoro is disappointed in them. However, remembering that Elena tried to protect them earlier, despite Marzel's demands that she listen to their parents and stay away from Elena and Nueva Vista, Marisa travels to the surface, and lures Elena to meet her by entrancing Elena with her singing. After revealing her ability to turn her tail into legs, Marisa asks for Elena's help with brokering a new peace and friendship between the human citizens of Avalor and the Sirenas.
Seeing that Marisa wants to be friends, Elena agrees to bring her cousin, Duke Cristobal, down to meet them, but when she does, Marisa is nearly captured by Cristobal's guards, but with Elena's help, Marisa, Marzel, and Coco escape, and Marzel reprimands his sister for thinking they could've trusted Elena. However, later on, Marisa is horrified when she sees Elena drowning after being disarmed of the Scepter of Light and knocked into the water during a confrontation with Shuriki and Fiero. Thinking quickly, she uses a special seaweed that she remembers is what their father used to save their mother by turning her into a Sirena, but despite Marzel's concern about turning Elena into one, Marisa is able to wrap it around Elena's torso in a way that enables her to breathe underwater, but not get turned into a Sirena, saving her from drowning. Afterwards, Marisa takes Elena back to Pescoro and Camila to see if she can get their help against Shuriki, but after revealing his fear regarding what happened to Marisa when she was younger, Pescoro refuses to help.
Marisa takes Elena back to Nueva Vista, but when she sees Elena get captured by a double-crossing Duke Cristobal, she tries to help, but is captured as well, while Marzel, retrieving Elena's Scepter of Light, goes to warn Pescoro of what happened. Marisa and Elena are locked up with Elena's family, but are busted out by Gabe, Mateo, Naomi, and the Jaquins. They then attempt to flee on the Royal Yacht, but are soon being attacked by Shuriki and her crew. Luckily for Marisa, Marzel warned Pescoro and Camila in time, and the Sirenas arrive to support Elena. After Marisa rejoins her family, they use their song to entrance the Duke's helmsman so he turns the Duke's yacht around to crash into the sea wall of his palace.
After Elena defeats Shuriki once and for all, Mateo petrifies Fiero again, and Victor and Carla Delgado escape with Cruz and Vestia, Marisa formally introduces her family to Elena's and Elena forges a newfound alliance and friendship between the human residents of Avalor and the Sirenas.
After the defeat of Shuriki, Marisa is happy to aid Elena with setting up a ceremony to have Elena and Pescoro officially sign a treaty that will secure the new alliance and friendship between the Sirenas and Avalor's human citizens. However, unknown to Marisa or Marzel, their Captain of the Guard, Daria, is against the treaty, as is Naomi. They are unaware of it as they help Elena plan the ceremony, despite Naomi's hostility against Marisa and her brother. Unfortunately, during the signing ceremony, one of the Sirenas arch-enemies, the malandros, disguised as a Sirena, disrupts the ceremony and ruins it for everyone. As Marisa and her family return to their Kingdom of Coronado, the malandros ambush them, having double-crossed Daria, who had made a deal with them to disrupt the ceremony in return for taking out the coral alarms, which the malandros could be repelled by, allowing them to take over Coronado. Horrified, Marisa returns to warn Elena, but when Naomi still shows contempt against Marisa, Elena demands an explanation for it, and learns that Naomi's grandfather had been on a ship that the Sirenas had sunk previously, hence her hatred of them. Despite Naomi still refusing to help them, Elena, joined by Mateo and Gabe, go with Marisa to save Coronado. After rescuing Daria, who apologizes for her part in making a deal with the malandros, which Elena forgives, they attempt to restore the coral alarms by having Elena regrow them with a spell from her Scepter of Light. Though she is later disarmed of it by the lead malandros, Saloso, she managed to restore the coral alarms in time to have them unleash their ringing, which the malandros can't bear to hear, forcing them to retreat in defeat, swearing they will return. Marisa is grateful to Elena for saving Coronado, and after Naomi agrees to let her grudge against them go and give them a chance, Marisa watches as Pescoro and Elena sign the treaty, and everyone celebrates.