Minami Kaido, a second-year Noble Academy student renowned as the "Academy Princess," embodies a proper, noble demeanor and strong sense of responsibility. She carries herself maturely, frequently exhibiting a caring, sisterly personality balanced by fairness and deep concern for others' well-being. Her initial ambition focuses on becoming a respectable person useful to others, later crystallizing into a specific aspiration to be a marine veterinarian.

Physically, she possesses sharp blue eyes and long teal hair styled with a portion braided across her head, hanging over her left shoulder. Her casual attire typically features a pale blue blouse with a lilac neck ribbon, a blue pencil skirt, and blue heels, varying seasonally to a blue summer dress or a light purple winter top paired with a long skirt.

Transformed as Cure Mermaid, her appearance shifts: ocean-blue hair forms a curled ponytail with dark ombre and purple streaks, accented by a gold tiara adorned with a pink clam and blue gem. Her costume consists of a blue top with scalloped sleeves, a pearl necklace centered by a gold sphere, and a sky-blue skirt incorporating fin-like tails, completed by white boots featuring scalloped trim. Mode Elegant variants introduce distinct themes: Ice showcases diamond and snowflake motifs; Bubble emphasizes spherical details and gradient effects; Sango incorporates shell and coral embellishments; and Halloween presents yellow flowers with layered ribbons.

Minami's relationships highlight her protective instincts. She initially corrects Haruka Haruno's tardiness but later bonds with her through ballet tutoring, recognizing Haruka's determination. Kirara Amanogawa addresses her casually as "Minamin," reflecting mutual respect despite differing opinions. A profound bond exists with Tina, the dolphin who saved her in childhood; when Tina suffers grave injury protecting her, Minami's distress activates the Miracle Bubble Dress Up Key for healing. She supports Towa Akagi during emotional struggles, defending her against adversaries. Childhood acquaintance Kimimaro Ijuin occasionally irritates her, particularly when interfering with her friendship with Haruka, though she accepts his apology after his rescue. She connects with Asuka Kitakaze over shared ocean interests, influencing her career path.

Within the "Pumpkin Kingdom's Treasure" movie segment, Minami participates in a Princess Convention, wins the dancing contest, but is subsequently trapped by the antagonist Warp. Cure Flora and Cure Scarlet later secure her freedom. Her actions contribute to restoring the kingdom's hope, aided by the Halloween Dress Up Keys. In "Cure Flora and the Mischievous Mirror," she admires Cure Flora's pumpkin-themed dress alongside Cure Twinkle and Cure Scarlet. For "Leff's Wonder Night!," she assists Princess Leff in confronting Night Pumpkin to restore light to the Night Kingdom.

Her backstory reveals a formative experience: discovering a washed-up Dress Up Key enabled her transformation into Cure Mermaid to defend Haruka from a Zetsuborg, solidifying her commitment to protection. A flashback depicts her childhood rescue by Tina, reinforcing her dedication to marine life. By the epilogue, she achieves her dream as a marine veterinarian, retaining a crystal replica of her Dress Up Key.