Rosalia de Catargena hails from the prestigious Catargena noble lineage, a family historically producing multiple queens. At seventeen, her refined manners, intelligence, and beauty positioned her as an exemplary candidate in the queen examination, with her lineage generating high expectations for success.

She represented the Royal Smallney Girls' Academy as one of two queen candidates, competing directly against Angelique Limoges. Their differing personalities initially sparked clashes during the examination period, where both utilized the powers of the nine Sacred Guardians to cultivate continents. This involved strategic requests to nurture their own continent or hinder the rival's progress.

Despite the rivalry, their interactions evolved beyond competition. Official events documented a growing mutual respect and cooperation, transforming their relationship into one marked by loyalty and support. Following the examination, Rosalia accepted the position of aide to the newly crowned queen, Angelique, regardless of the outcome. She served in this role across multiple cosmic realms, including the Sacred Bird Cosmos and Shinchō universe, providing both administrative and moral support.

Rosalia enjoyed royal milk tea, pear charlotte desserts, opera performances, and playing the violin. She disliked coarse men and specific foods like mushrooms and anchovies. Across adaptations and spin-offs, her core characterization remained consistent: a dignified noblewoman whose initial rivalry softened into a steadfast alliance, serving alongside Angelique in cosmic governance.