Monica Lux, younger sister of Allu Lux, originally served alongside her as rhythmists in the capital’s military, crafting Echo Devices for warfare. The sisters abandoned dreams of joining the Court Orchestra to fund treatment for their mother’s blindness. After Rin healed their mother using the Song of Healing, Monica and Allu joined Rin’s group journeying to the capital.

Monica wields exceptional musical talent, mastering perfect auditory mimicry with a single hearing—replicating any song or sound flawlessly. This ability amplifies the effects of Rin’s spirit songs during joint performances, with Monica’s vocal strength focused on harmonization to enrich compositions.

Her quiet, reserved nature contrasts with Allu’s assertiveness. Prone to extreme anxiety, Monica involuntarily falls asleep when confronting fear or danger, complicating escapes or conflicts and necessitating aid from companions.

Visually, she stands out with auburn hair and pink eyes, perpetually clad in a full-body pink zippered rabbit suit.