Aika S. Granzchesta is a sixteen-year-old apprentice Undine working for the Himeya Company in the city of Neo-Venezia on the planet Aqua, formerly known as Mars. As the only daughter of the family that owns Himeya, one of the oldest and most prestigious water-guide companies in the city, she is also the heiress to the business. Her birthday is on February 2nd. She has amber eyes and dark blue hair. Initially, she wears her hair in two long, thin braids, a style given to her by her idol, Alicia Florence. After an accident burns much of her hair, she cuts it into a shorter pixie-style bob secured by four hairpins on the left side, a look she retains as she matures; as a young adult, her hair grows back and is often styled in a small bun with front strands reaching her upper chest. She wears the Himeya uniform of white and red, along with spherical red earrings and a small rounded hat bearing the company logo.

Aika is known for her blunt, pragmatic, and competitive personality. Her famous catchphrase is "Hazukashii serifu kinshi!" meaning "Embarrassing remarks are not allowed!" which she frequently directs at her more idealistic friend, Akari Mizunashi. Despite her tough exterior, she is deeply emotional and prone to tears, often called a crybaby by her friends. She takes great pride in her skills as a gondolier and works relentlessly to improve, fearing that any success might be dismissed as nepotism due to her family name. Her primary motivation comes from a childhood encounter when Alicia Florence, then an apprentice herself, comforted her with a gondola ride. This inspired Aika to become an Undine, and she openly idolizes Alicia. However, her family obligations tie her to Himeya, preventing her from joining the Aria Company as she once wished.

Aika forms close friendships with Akari Mizunashi, her first friend on Aqua, and Alice Carroll, despite their contrasting personalities. Her mentor is Akira E. Ferrari, one of the legendary Three Water Fairies. Though Aika often complains that Akira is too strict, she secretly respects her deeply, recognizing that Akira treats her as a normal trainee rather than the boss's daughter. She also harbors romantic feelings for Albert Pitt, a Gnome who works in the city's underground. Over the course of her journey, Aika undergoes significant personal growth. She passes her challenging Prima examination, earning the title Rosen Queen (Crimson Rose), and learns to reconcile her family heritage with her own identity. As an adult, she becomes the confident manager of a new Himeya branch, retiring an antique gondola and introducing new traditions such as seasonal items at the Himeya-affiliated Rose Garden café. She is a skilled and dedicated Undine, known more for her persistent hard work than for innate talent, and she also proves to be a proficient cook.