Minami Kaidō, known as Cure Mermaid, serves as one of the main protagonists. This 14-year-old second-year student at Noble Academy holds the position of student council president, earning admiration as the "Academy Princess" for her poise, elegance, and strong sense of responsibility. Her family manages a seaside resort business, and Minami initially planned to inherit this role before embracing her true passion for marine biology and pursuing a future as a marine veterinarian.

She possesses long teal hair, typically styled with a braid over her left shoulder, and blue eyes. Her civilian attire reflects her dignified persona, featuring refined outfits like pale blue blouses with lilac ribbons, pencil skirts, and heels. Minami harbors a childhood fear of ghosts. Her personality blends strictness with deep compassion; she acts as an older sister figure, prioritizing others' safety and well-being. She initially struggles to express personal dreams, fearing family disappointment, but gradually learns to voice her aspirations.

Discovering a Dress Up Key on the shore enables Minami to transform into Cure Mermaid, the Princess of the Sea. Her transformation activates using the Princess Perfume and the phrase "Pretty Cure Princess Engage!" In her Cure form, her hair turns ocean-blue with purple streaks, adorned with a gold tiara featuring seashell motifs. She wears an elaborate blue-and-white outfit embellished with pearls, incorporating fin-like elements and a mermaid-tail-inspired skirt. Minami wields multiple Dress Up Keys (Mermaid, Ice, Bubble, Sango), enabling distinct attacks: **Mermaid Ripple** (a water-based purification technique), **Frozen Ripple** (an ice attack via the Ice Key), **Bubble Ripple** (protective/healing bubbles via the Bubble Key), and **Coral Maelstrom** (a coral-themed offensive via the Sango Key). She utilizes the Crystal Princess Rod for enhanced abilities and accesses Mode Elegant forms—alternate gowns like Ice, Bubble, Sango, and Royal for tactical or ceremonial scenarios such as underwater combat or royal events.

Key relationships shape Minami's journey:
* **Haruka Haruno (Cure Flora)**: Minami is initially stern towards Haruka for tardiness but softens after tutoring her in ballet. Witnessing Haruka's determination, Minami risks herself to protect Haruka in battle, triggering her transformation. They become confidants, with Minami later selecting Haruka as a dance partner.
* **Kirara Amanogawa (Cure Twinkle)**: Kirara uses the casual address "Minamin," signaling mutual respect despite differing opinions.
* **Tina**: A dolphin who rescued Minami in childhood. Their bond deepens when Tina shields Cure Mermaid in battle, nearly dying. This trauma unlocks the Miracle Bubble Key, which heals Tina.
* **Towa Akagi (Cure Scarlet)**: Minami aids Towa in overcoming nightmares and defends her identity against antagonists, reinforcing their trust.
* **Kimimaro Ijuin**: A childhood acquaintance whose interference in Minami’s friendship with Haruka strains their relationship until reconciliation.
* **Asuka Kitakaze**: Inspires Minami’s career shift to marine biology through their shared love of the ocean.

Minami's development involves significant growth. Early episodes depict her as perfectionistic and self-sacrificing, but she gradually embraces vulnerability. A pivotal moment occurs when she confesses her dream to her parents, leading to tearful acceptance. By the series' conclusion, she achieves her goal as a marine biologist, seen in adulthood wearing a lab coat with a braided ponytail. Within the team, Cure Mermaid represents the "sea" in the Land-Sea-Sky motif alongside Cure Flora (land) and Cure Twinkle (sky). Her battle catchphrase is: "You’ve locked a dream away in a frigid cage... I intend to retrieve it! Have you prepared yourself?"