Rosina was a gray shrine maiden apprentice in the Ehrenfest temple, initially serving Sister Christine. Her exceptional talent with the harspiel earned her significant privileges under Christine's patronage, including exemption from physical chores to protect her hands. This specialized treatment led her to mistakenly believe it was standard for female noble retainers, shaping her early expectations.

Ferdinand assigned Rosina as Myne's attendant and music teacher, a role she eagerly accepted to continue her musical pursuits. Conflicts arose with Myne's head attendant, Fran, due to Rosina's refusal to perform non-musical tasks like paperwork and chores, which she considered beneath a blue shrine maiden's retainer. Myne confronted this discrepancy, issuing an ultimatum: perform standard duties or return to the orphanage.

After consulting fellow gray shrine maiden Wilma, Rosina realized Christine's approach had been atypical and Myne's expectations aligned with standard temple practices. She chose to remain as Myne's attendant, taking on responsibilities like assisting Fran with paperwork while continuing musical instruction. Myne accommodated her artistic focus by minimizing tasks risking hand injuries and restricting late-night instrument practice to avoid disturbances.

Rosina collaborated with Myne to transcribe songs from Myne's memories of her past life as Urano Motosu. This transcription became Rosina's passion, and she treasured each new melody. Recognizing this dedication, Myne gifted her the collection of transcribed songs during Rosina's coming-of-age ceremony. Following Myne's transition to becoming Rozemyne, Rosina was formally appointed as her personal musician, leaving her orphan status to serve exclusively in this role.

Rosina later accompanied Rozemyne to the Royal Academy, where her musical performances gained prominence. When Rozemyne's compositions attracted attention from music professors, Rosina performed them at a tea party attended by professors, royalty, and the second prince. Though her former mistress Christine intended to reclaim her after graduation, Rosina reaffirmed her commitment to serving as Rozemyne's musician following a discussion between them.

Beyond music, Rosina demonstrated skill in painting and calligraphy, noted for her elegant handwriting. Her graceful demeanor and refined conduct gave an impression of nobility. However, her intense focus on music sometimes caused obliviousness to other matters, requiring explanations for non-musical tasks or social nuances.