Sonia Yanagi, a first-year high school student sharing Kana Fujii's school, wields considerable healing powers. She acquired C-level healer certification via an overseas accelerated program before her introduction, placing her beyond the main apprentice trio at Karasuma Voice Treatment Clinic in skill. She grew up abroad, specifically noted as half-Russian, though her family details and specific countries remain undefined.

Driven by training under Shinobu Honosaka's grandmother—the same mentor who taught renowned healer Ria Karasuma—Sonia initially adopts a competitive stance, viewing Ria as her professional rival, establishing a generational link in their contention. Her demeanor reflects confidence and occasional arrogance, traits that gradually soften. Her closest bond is with childhood friend Shinobu Honosaka, an aspiring "sonic musician" creating healing music, who consistently affirms Sonia's inherent kindness beneath her competitive exterior.

Sonia's interactions shift from rivalry to collaboration with Kana, Hibiki, and Reimi. This evolution culminates in her joining them for the C-rank healer certification exam, which the group collectively passes by the series' end. Her character development traces a path from a solitary rival to an integrated member of the healer group, demonstrating her capacity for teamwork and mutual growth, particularly evident in later joint performances and training sequences. While her personal backstory receives less focus than the main trio's, her role provides contrast to their apprenticeship journey and underscores international pathways within the healing profession. Her narrative function supports themes of collaboration, professional growth, and diverse approaches to vocal medicine.