Monica Lux, a timid young girl draped in a pink bunny-hooded cloak, journeys alongside her bold older sister Allu as rhythmists in the king’s military. Together, they harness their musical gifts to bolster soldiers’ morale, driven by a shared mission to fund a cure for their mother’s blindness—a goal realized when Monica aids Rin in performing the Song of Healing.

Monica’s rare auditory gift allows her to flawlessly replicate any sound or melody after a single hearing, a skill pivotal in magnifying the power of others’ music, particularly Rin’s. Overwhelmed by fear, she withdraws into her hood’s shelter, often dozing off and drawing curious stares from onlookers who mistake her for an odd creature. Though her quiet reserve contrasts with Allu’s confidence, their sisterly bond and devotion to family remain unshakable.

Once their mother’s sight is restored, the sisters forsake aspirations to join the Court Orchestra, opting instead to accompany Rin’s group toward the capital. Monica’s harmonies and rhythmic precision continue to anchor the ensemble’s musical strategies, her contributions growing more assured over time. While she evolves from relying on Allu to engaging more independently within the group, her timid demeanor and mimicry prowess endure as central traits, quietly shaping her role in their collective journey.