Ai Aino originates from Manhome (formerly Earth) and initially perceived the slower pace of life on Aqua (formerly Mars) as uninteresting. Her encounters with Akari Mizunashi during visits to Neo-Venezia gradually cultivated a deep appreciation within Ai for Aqua's subtle beauty and unique charm. This shift in perspective forged a close friendship with Akari, sustained through near-daily email correspondence.

Despite residing on distant Manhome, Ai frequently journeys to Aqua, often accompanied by her parents or older sister. These trips enable her to deepen her connection to the planet and strengthen her bond with Akari. Ai's growing admiration for Akari and the Undine profession ultimately leads her to become Akari's apprentice at Aria Company.

As an apprentice, Ai trains under Akari at Aria Company alongside fellow trainees Azusa and Anya. This apprenticeship signifies a key transition from occasional visitor and correspondent to dedicated student learning the skills and traditions of an Undine.

Within the series, Ai serves as the narrative recipient of Akari's reflective emails in the anime adaptation, providing a thematic structure. Her character arc demonstrates how exposure to Akari's worldview fosters her evolving relationship with Aqua, transforming initial indifference into active cultural participation through her apprenticeship.