Aika S. Granzchesta is the sole daughter and heir to the proprietors of Himeya Company, Neo-Venezia's oldest water-guide corporation on the terraformed planet Aqua. Her family's prominence initially unsettles her, as she perceives excessive deference from employees, fueling resentment toward her inherited legacy. This struggle intensifies during her youth as she seeks an identity distinct from Himeya's expectations. Her outlook transforms upon witnessing Alicia Florence, whose elegance and prowess as an undine motivate Aika to embrace gondolier training despite early reservations about her birthright.

Aika possesses amber eyes and long hair, first worn in slender twin braids extending to mid-thigh as ponytails. She dons Himeya's signature red-and-white uniform with spherical red earrings, a rounded hat bearing the company emblem, and gold-accented red heels. Seasonal adaptations include a summer ensemble with a cropped blouse and ankle-length underdress featuring twin red arrows on the back, plus a winter variant comprising a cape-style top, long sleeves, and gray tights. After a fire damages much of her hair, she adopts a pixie cut with two short front strands—the left fastened by four pins. In adulthood, her regrown hair settles into a small crown bun, front strands reaching her upper chest and pinned.

Blunt and forthright in speech, Aika habitually interrupts Akari Mizunashi's sentimental declarations with the rebuke "Embarrassing remarks are not allowed!" Despite this brusqueness, she forges deep bonds with Akari and Alice, though her similar temperament sparks friction with her mentor, Akira E. Ferrari. She takes pride in reliability and competitive undine skills yet reacts tearfully to emotional moments, earning her friends' "crybaby" moniker. Her reverence for Alicia drives frequent visits to Aria Company, where she openly wishes to join if not for familial duty. She nurses romantic feelings for Al, a Salamander apprentice in climate control.

Her growth focuses on conquering self-doubt about abilities separate from her lineage. Training under Akira instills the principle that success derives from persistent effort over innate talent, which Aika adopts. This conviction peaks during her Prima undine examination, where she demands an exceptionally arduous trial to validate her individual merit. The midnight test requires retrieving a rose from Akira without leaving her gondola before dawn. Utilizing strategic shortcuts and polished techniques honed through rigorous practice, Aika triumphs, cementing her evolution into a resilient, self-assured undine. Her promotion grants the title "Rosen Queen," signifying newfound confidence.

As a Prima, Aika navigates preserving Himeya's traditions while carving her own legacy. She initially balks at inheriting the company's last founding-era gondola, seeing it as a burdensome relic rather than a testament to her hard-won skills. After reconciling with Akira and reflecting on her journey, she assumes leadership of a new Himeya branch. Innovations like seasonal café menus showcase her dedication to merging heritage with progress. Her shift from reluctant heir to proactive leader crystallizes when she mentors her apprentice, Azusa.

Relationships anchor Aika's narrative. Her rapport with Akira matures from conflict to profound trust, grounded in Akira's steadfast faith in her potential. While idolizing Alicia, she privately cherishes Akira's unwavering guidance. Her camaraderie with Akari and Alice offers mutual support throughout their careers. Later, she sustains these ties while juggling mentorship and corporate responsibilities.