Roux Ltskin stands as Firiel Dee's steadfast childhood companion and primary protector. He works as assistant and student to Firiel's father, the astrologer Gideon Dee, within their mountain observatory base for scientific research—including forbidden studies drawing dangerous attention. His background remains shrouded in secrecy early on, marked by personal struggles that forge emotional distance from Firiel. He perceives his own history as shadowed and incompatible with her radiant purity, stating she "shines like the sun itself."

Initially interacting with Firiel in a brotherly fashion, Roux harbors deeper romantic feelings. His protective instincts surge when Firiel is revealed as a royal heir and thrust into political conflict. He consistently intervenes against threats, including attacks by groups like the Snake Rod where their guardian, Bo Holy, dies defending them. His motivations blend fierce loyalty to Firiel with a distinct aversion to nobility, fueling his cynical view of court politics.

As political tensions escalate, Roux faces capture and torture by antagonistic nobles seeking leverage over Firiel. Imprisoned, he prioritizes safeguarding Firiel above his own suffering, refusing to cooperate with captors like Leandra Cheviat despite blackmail attempts. This ordeal intensifies his protective resolve while deepening the emotional complexity of his bond with Firiel. Their dynamic shifts gradually from familial affection to mutual romantic acknowledgment, culminating in a committed partnership. They ultimately leave court life together, returning to their original home as a couple.

Personality-wise, Roux demonstrates analytical thinking and prefers observation before action, aligning with descriptions of him as introspective and intellectually driven. He exhibits occasional social detachment yet maintains unwavering loyalty, particularly towards Firiel. His scientific curiosity and disdain for aristocratic manipulation remain consistent throughout his journey.