Marin is a mermaid who transforms into human form when her tail dries. Driven by her romantic fixation on Riiya, a werewolf boy, she frequently clashes with Chacha over their closeness. Introduced during a beach excursion, her vanity and self-centered tendencies surface as she prioritizes her whims over others’ needs. Though enrolled in the Apple Class under Teacher Barabaraman, she gravitates toward Banana Class students, forming situational alliances with Yakko and Orin. Her dynamic with the group balances comedic rivalry—particularly her resentment toward Chacha—and reluctant cooperation against shared threats like Daimaō’s forces.

Unaware of Riiya’s werewolf nature, she mistakes his transformed state for a dog, fueling humorous misunderstandings. Her role oscillates between antagonism and collaboration, dictated by circumstance. Traits like possessiveness, dramatic outbursts, and vanity spark recurring comedic tension, especially in scenarios involving romantic competition or misunderstandings. Beyond her mermaid origins, no additional background details are provided, and her narrative focus remains fixed on unrequited feelings for Riiya and rivalry with Chacha, without substantial evolution outside these core dynamics.