Cateau hails from the Flower Star, a magical realm inhabited by plant spirits. Dispatched to Earth as one of two messengers, their mission is to locate a descendant of the Flower Star people capable of finding the legendary Seven-Color Flower, essential for choosing the planet's new ruler. To operate discreetly on Earth, Cateau assumes the form of a blue-eyed talking cat, while the other messenger, Nouveau, takes the form of a dog. Though fundamentally a winged humanoid Flower Spirit, Cateau sometimes exhibits feline mannerisms or sounds while in their animal disguise.

Identifying Lunlun in France as the sought descendant through her maternal lineage, Cateau and Nouveau recruit her for the quest. Guiding and protecting Lunlun across Europe and parts of Northern Africa, Cateau provides companionship and magical assistance. They help her overcome challenges posed by antagonists Togenishia and Yabouki, who seek the flower for their own ambitions. Cateau participates in all key events, including a critical incident in Switzerland where Lunlun suffers a near-fatal fall, destroying her initial magical pin. Following Lunlun's revival and receipt of a new, life-linked pin, Cateau continues supporting her mission consistently.

In the OVA *Konnichiwa Sakura no Sono*, Cateau accompanies Lunlun to Japan. There, they assist in confronting Togenishia and Yabouki, who cause cherry blossoms to wither prematurely through pollution-related sabotage, aiding efforts to protect the blossoms.

Cateau shares a close, sibling-like dynamic with Nouveau, working in tandem to advise and safeguard Lunlun. Their relationship with Lunlun is defined by loyalty and mentorship, consistently prioritizing the mission's success. Cateau undergoes no significant personal evolution or standalone background development across the series or OVA, remaining a steadfast magical companion focused solely on the central quest.