Allu Lux, older sister to Monica Lux, shares their abandoned dream of joining the imperial Court Orchestra. Both sisters instead became rhythmists for the king's military, driven by the urgent need to earn money for medical treatment to restore their mother's eyesight—a primary motivation fueling their early story involvement.

Allu possesses striking red hair and matching red eyes. She typically wears a bra, sleeveless vest, and pants, an outfit noted for its revealing nature, complemented by youthful freckles.

Extroverted and talkative, Allu is quick to engage in conflicts but equally quick to step in and resolve the disputes of others. This contrasts sharply with her younger sister Monica's shy and quiet nature, establishing a dynamic where the outspoken Allu acts as Monica's protector. Allu demonstrates deep care for her family, particularly Monica, and bravery in challenging situations.

Her musical background persists as she habitually creates rhythmic beats by hitting her hands on various objects or people, reflecting her role as a percussionist without a formal instrument.

Allu participates in military performances to boost morale while crafting echo devices. After Rin uses the Song of Healing, aided by Monica's vocal harmonization, to cure their mother's blindness, Allu and Monica leave the military. They join Rin and her companions on their journey to the capital, integrating into the main group's efforts.

During this journey, Allu readily engages in physical confrontations when necessary. She incapacitates a soldier with a wrestling move after Pony Goodlight initiates an attack, showcasing her willingness to protect allies. Her protective instincts are most evident in her consistent shielding of Monica from threats.

Following their mother's healing and the resolution of the central conflict, Allu continues traveling with the group as part of the ensemble focused on rebuilding and moving forward.