Monica Lux is the younger sister of Allu Lux. She journeys alongside her sister as a rhythmist in the king's military, boosting troop morale through musical performances. This work funds their goal of restoring their mother's eyesight. With Monica's aid, Rin performs the Song of Healing, successfully curing their mother's blindness. Afterwards, both sisters accompany Rin and her companions to perform in the capital.

Monica wears a distinctive pink bunny-hooded cloak. When frightened or threatened, she habitually pulls the hood low over her head and falls asleep. This often leads others to mistake her for a strange creature. Her tendency to sleep during perilous situations requires rescues by the group and recurs throughout various events.

Musically, Monica demonstrates prowess in drumming, maintaining rhythm, and singing. A notable skill is her ability to flawlessly reproduce any music or sound after hearing it just once. This mimicry amplifies the strength and effects of Rin's spirit songs during performances and battles. Her vocals frequently enhance the group's musical output.

Personality-wise, Monica is shy and less talkative, contrasting with her extroverted and protective older sister, Allu. This establishes a dynamic where Allu often defends Monica, sometimes exhibiting a harsh, occasionally brusque manner. Monica's unique abilities and personality traits contribute to the group's dynamics and challenges throughout their travels.