Pipi is a young mermaid of the Triton clan and, alongside Triton, one of only two surviving members of her people in the anime. In the original manga the same character is given the fuller name Pipiko, also written Pipico, while the anime refers to her simply as Pipi. She is introduced in the North Sea, where the old seal Proteus has raised her in hiding since she was very young, sheltering her from the Poseidon clan that destroyed the rest of the Triton family.

Because she grew up protected and indulged, Pipi develops a temperamental and strong-willed personality. She is proud, stubborn, and quick to argue, and she clashes with Triton from the moment they meet. She resents the attention Triton receives as a hero among the seals, dislikes being told what to do, and refuses to accept his company willingly. Her rivalry with him leads to serious consequences: her own reckless behavior allows the Poseidon clan to capture her, and the seals and Proteus are killed in the attempt to rescue her. Afterward she has no choice but to travel south with Triton, though she remains emotionally closed off and cannot forget the North Sea she has lost. She even runs away from Dolphin Island by herself and is attacked by the giant sea serpent Ryuda before Triton comes to her aid.

Her motivation is not initially revenge. What drives her is grief for the home and caretakers she has lost, along with a defensive pride that makes her resist depending on Triton. Over the course of the journey, however, her reason for staying changes. When Poseidon destroys Dolphin Island, Triton and Pipi are left alone together in a desperate situation, and it is at that point that she finally opens up to him. From then on she stands beside him as a true companion, sharing the dangers of his struggle against the Poseidon clan instead of resisting him.

Within the story, Pipi serves as Triton's closest companion and the living proof that his people are not entirely gone. Her presence gives him someone of his own kind to protect and fight for, and the two of them, along with the dolphins Ruka, Iru, Karu, and Fin, face repeated attacks by Poseidon's forces. Her most important relationships are with Proteus, whose death marks the end of her sheltered childhood; with Triton, whose bond with her evolves from constant bickering into mutual trust; and with the dolphin companions who travel alongside them. Her enemies are the Poseidon clan and the various sea creatures they send after her and Triton.

As a mermaid of the Triton clan, Pipi is naturally suited to life in the ocean and is able to travel freely through the open sea. The anime does not give her any special combat powers or supernatural abilities; her role is defined instead by her determination, her spirited temperament, and the bond she eventually forms with Triton. Her defining quality is her personal growth, from a spoiled and lonely girl into a steadfast partner who chooses to face danger at Triton's side. In the manga version of the story, the character, there named Pipiko, follows a longer arc and eventually matures into an adult, marries Triton, and becomes the mother of seven children named after the colors of the rainbow, though that later married life belongs to the manga rather than to the anime's portrayal of the young Pipi.