Princess Marina, known as Rei dos Mares, is the protagonist of the 1975 Japanese animated adaptation of Hans Christian Andersen's fairy tale. She is the youngest daughter in the royal merman family, residing in an undersea kingdom with her father and grandmother. Marina's defining trait is her intense curiosity about the human world above the sea, leading her to scavenge surface artifacts. She possesses blonde hair, a mermaid tail, and the kingdom's most beautiful voice. Her coming-of-age ceremony, contingent on demonstrating reliability, involves receiving a pearl hairpin from her grandmother. Marina initially struggles with this responsibility due to her rebellious nature. Her closest companion is Fritz, an Atlantic dolphin calf, sharing adventures and secrets. During an excursion, Marina discovers a statue of a human prince in a sunken ship, deepening her fascination with humanity. Defying prohibitions against surfacing before adulthood, she witnesses the human prince aboard a ship during a storm. When he falls overboard, she rescues him and brings him ashore, where an unidentified raven-haired princess finds him. This act of bravery convinces her grandmother to grant her the pearl hairpin, recognizing her maturity. Driven by love for the prince and a desire to gain an immortal soul—a trait mermaids lack—Marina visits the Sea Witch, depicted as a shrewd devil ray who conjures storms. In exchange for a potion granting human form, Marina surrenders her voice and accepts irreversible transformation. The Sea Witch warns that failure to marry the prince will result in Marina dissolving into sea foam at dawn after his wedding to another. After an emotional farewell to her family and Fritz, Marina consumes the potion and becomes human. Discovered by the prince on the shore, Marina lives in his castle for a month. They grow close, and he confides his wish to marry her instead of a foreign princess arranged by his parents. This intention triggers jealousy in the prince's cat, Jemmy, who manipulates events to frame Marina for treason. When the foreign princess arrives—revealed as the same raven-haired woman who found the prince—he marries her, sealing Marina's fate. In despair, Marina prepares to die, but her sisters intervene, having traded their hair to the Sea Witch for a magical dagger. They instruct Marina to kill the prince and let his blood touch her feet to regain her mermaid form. At dawn, Marina enters the prince's chamber but cannot commit the act, choosing instead to kiss him farewell. She discards the dagger into the sea, awakening the prince, who realizes her identity too late. As the sun rises, Marina dissolves into foam and ascends to the sky, becoming a spirit of the air. Her essence ultimately merges with the sea. Her background and development are confined to this narrative.