Sphinx acts as the demonic companion and weapon of Nagahisa Kai, the kidnapped half-brother of protagonist Setsuna Kai, forced into the demon realm early in the story. Though Nagahisa carries angelic lineage and harbors ingrained contempt for demons, he depends solely on Sphinx—a paradoxical bond forged through Tyrant Isis’s calculated “mercy.” This alliance fuels his inner turmoil, juxtaposing his ideological rejection of demons with his reliance on their power.

Nagahisa’s discovery of his angel-child identity triggers a defiant repudiation of his brother and demonkind, yet he continues wielding Sphinx, exposing the friction between his professed ideals and ruthless pragmatism. The partnership evolves into a persistent emblem of his unresolved conflict, mirroring his fractured identity and the morally ambiguous battleground of angels versus demons.

Though Sphinx’s motives and traits remain unexplored, its unwavering presence amid Nagahisa’s descent into antagonism amplifies themes of coerced loyalty, systemic prejudice, and the volatile interplay of enmity and dependence in a fractured world.