Dia Cadence is a bioroid from Poseidon, a maritime industrial nation distinct from the central city-state Olympus. Her existence confirms bioroid production extends beyond Olympus' borders, challenging assumptions of their exclusive creation there. Dia first appears during a terrorist incident in Olympus, where Briareos saves her from falling debris during an ESWAT operation, establishing her initial link to the protagonists.

After this event, Dia resides with the Olympus-based bioroid Hitomi, signaling her status as a newcomer to the city. This living arrangement integrates her into the protagonists' social circle. When Deunan Knute arrives intoxicated at Hitomi's residence seeking refuge after an argument with Briareos, Dia facilitates communication by contacting Briareos, contributing to their reconciliation.

Dia's Poseidon origin broadens the series' exploration of human-bioroid relations, demonstrating the wider geographical distribution of bioroid technology and society. Specifics of her creation or purpose in Poseidon remain undocumented. Her narrative function connects the protagonists to broader political dynamics through her origin and catalyzes character interactions during early events.