Minami Kaido, known as Cure Mermaid, is a central character in Go! Princess Pretty Cure. She is a poised and elegant 14-year-old, serving as the student council president at Noble Academy where she holds the title "Academy Princess." Her family runs a seaside resort, and Minami initially felt obligated to inherit the business, suppressing her personal dream of working with marine life. This conflict caused her deep distress, fearing she would disappoint her family while yearning for her passion. Her strong sense of responsibility and caring nature often make her protective of others, though she can be strict. Despite her composure, she harbors a fear of ghosts.

Minami discovered her destiny as a Pretty Cure after finding a Dress Up Key on the shore. She transformed into Cure Mermaid to protect Haruka Haruno (Cure Flora) from a Zetsuborg, driven by her determination to safeguard the academy. As Cure Mermaid, the Princess of the Sea, she commands water-based powers. Her transformation features ocean-blue hair with purple streaks in a flowing ponytail, a seashell tiara, and an aquatic-themed costume with pearl accents and fin-like skirt details. She wields the Princess Perfume and later the Crystal Princess Rod. Using specific Dress Up Keys (Mermaid, Ice, Bubble, Sango), she executes attacks like Mermaid Ripple, Frozen Ripple, Bubble Ripple, and Coral Maelstrom. Her "Mode Elegant" forms alter her attire with ice diamonds, bubble gradients, or coral motifs, enabling group attacks like Trinity Luminere and Eclat Espoir.

A defining moment involved her childhood friend, a dolphin named Tina, who sacrificed herself protecting Minami in battle. This triggered the Miracle Bubble Dress Up Key, which healed Tina and strengthened Minami's resolve to protect others. Relationships shape her journey: she forms a close bond with Haruka after initially mentoring her in ballet; supports Kirara Amanogawa (Cure Twinkle) despite differing views; and aids Towa Akagi (Cure Scarlet) in overcoming trauma, fiercely defending her identity. She also reconciles with her brother, Wataru, and childhood acquaintance Kimimaro Ijuin, whose interference in her friendship with Haruka she firmly resisted.

Minami evolves by embracing her dream of becoming a marine veterinarian, inspired by marine biologist Asuka Kitakaze. This marks a shift from familial obligation to self-determination. As an adult, she works as a marine biologist, seen wearing a lab coat with her hair in a braided ponytail. Her story centers on responsibility, compassion, and pursuing true aspirations.