Mari Yumenokata lives next door to Gou Takarada in a large estate, his neighbor and childhood friend. She nurses a secret, unrequited affection for Gou, a source of frustration since he remains oblivious. Mari stands as Gou's only steadfast companion, regularly joining his treasure hunts despite others labeling him eccentric. Her loyalty makes her his primary emotional support during their adventures.

She is the daughter of Manzou Yumenokata, founder and director of the Yumenokata Foundation, an organization engaged in large-scale development. This connection creates tension when Gou's treasure hunting clashes with her father's secret corporate projects. Mari possesses a notable fondness for bathing, owning her own bathing pool. For her birthday, her father gifts her a trip to a unique new public bathhouse facility he developed.

During the hunt for the legendary Salamander and the Four Treasures of Heart, Mari encounters and befriends Leen Kyami Kuunyo, a fairy and witch. Leen unexpectedly supports Mari's attempts to gain Gou's romantic notice, providing her with an ally outside Gou's influence. This treasure hunt causes significant disruptions, particularly to her father's giant bathhouse project designed as a phoenix structure with transparent bathtubs suspended from its wings. This project becomes central to the chaotic events of the second episode.

Mari participates in a bathing sequence with Leen in the bathhouse, showcasing their bond. The structure malfunctions, transforming into a flying phoenix that catapults the occupied bathtubs into the air, exposing the bathers. Her involvement highlights her role in both the comedic and narrative elements of these events.