Karen Minazuki serves as the third-year student council president at L'École des Cinq Lumières. Raised primarily by her butler, Sakamoto, due to her parents' frequent international tours—her father a pianist and her mother a violinist—she resides in a large estate with extensive grounds including a greenhouse, stables, and private villas. This upbringing fostered a significantly different understanding of financial norms compared to her peers. Childhood loneliness from missing her parents was carefully concealed.

Her close friendship with Komachi Akimoto began in their first year of middle school. When a cancelled factory tour threatened a class project, Komachi's encouragement spurred Karen to persist; together they secured an alternative venue, forging an inseparable bond. Komachi later became Karen's fellow council member and closest confidante.

Initially skeptical of Pretty Cure, Karen witnessed a Nightmare organization attack at her home. Though a blue butterfly initially eluded her, preventing transformation, she later became Cure Aqua when her desire to protect friends overcame self-doubt. Her transformation phrase is "The blue spring of intelligence, Cure Aqua!" She represents the "Pretty Cure of Intelligence," wielding water-based powers.

As Cure Aqua, her attacks include **Aqua Stream** (a pressurized water stream), **Aqua Tornado** (an enhanced whirlwind using the Aqua Ribbon accessory), and **Sapphire Arrow** (water-based projectiles). She demonstrates proficiency in hand-to-hand combat and occasionally wields the Aqua Ribbon as a blade. She contributes to team attacks **Five Explosion**, **Rainbow Rose Explosion**, and **Floral Explosion**. Power-up forms include **Super Cure Aqua**, featuring butterfly wings and enhanced abilities accessed via Miracle Lights, and **Cure Rainbow Aqua** from crossover films, incorporating feather-like motifs and angelic elements.

Karen maintains a calm, stoic demeanor, admired as a model student. This masks underlying loneliness and a tendency to shoulder responsibilities alone, stemming from her parents' absence and student council role. Early as a Pretty Cure, she struggled with emotional openness, sometimes lashing out at Komachi under stress. Over time, interactions with teammates—especially Nozomi Yumehara's persistent friendship—helped her become more expressive and trusting. Sakamoto noted her increased willingness to joke and show vulnerability.

Her relationship with Rin Natsuki began contentiously, marked by frequent arguments due to opposing personalities (Karen's intellect vs. Rin's passion) and elements (water vs. fire). Through battles and collaboration, they developed mutual respect and camaraderie. Karen also formed a protective, sisterly bond with Milk (Kurumi Mimino); Milk's illness inspired Karen's aspiration to become a doctor. This goal defined her adult life, working as a physician and showing empathy toward patients like the initially distrustful Nana Shinozaki.

In her civilian identity, Karen excels academically and athletically, skilled in horseback riding, tennis, and water skiing. She exhibits eccentricities like a pronounced fondness for grape juice and participating in eating contests, yet dislikes small animals like lizards and struggles with mundane tasks like cooking.

As an adult doctor, she wears a white coat and ponytail, maintaining her compassionate yet responsible nature. She occasionally revisits past adventures with the Pretty Cure team, reflecting on their shared growth.