Deia Chades, also known as Dia Cadence, hails from the maritime industrial nation of Poseidon and works as a scientist specializing in bioroid research. She first appears during a terrorist incident in Olympus, nearly crushed by falling debris during an ESWAT operation before being rescued by Briareos. Later, she visits Hitomi's residence and is introduced to Deunan Knute as a visitor from Poseidon.

Deia was married to Alcides, leader of the nomadic shipborne society called the Argonauts. The emergence of a bioroid clone of Alcides triggers intense jealousy within her. This emotional turmoil leads her to betray the Argonauts by revealing the location of their mobile fortress-city to Poseidon. Poseidon launches a missile attack that destroys the fortress. Alcides dies protecting Deia during the assault. Overwhelmed by guilt and grief, Deia descends into severe psychological instability.

Her mental breakdown manifests in a campaign of terrorism against Olympus. She covertly pilots a giant Hecatonchires-class Landmate, using it to carry out violent attacks aimed at sabotaging Olympus's societal stability and its Ark Project, an initiative to preserve humanity under bioroid guidance. Her primary motive is to destroy all "unprocessed" bioroids carrying Alcides's genetic material, viewing them as abominations perpetuating her trauma.

This vendetta culminates in a confrontation where she prepares to kill a newborn infant bearing Alcides's genes. At the critical moment, she experiences a moment of clarity and halts her attack. Accepting responsibility for her actions and seeking redemption, Deia commits suicide via self-immolation.