Gunzō Chihaya, a former student of the Japanese National Marine Academy, assumed command of the privateer group Blue Steel. He captains the Fleet of Fog submarine I-401, which defected to humanity alongside its humanoid avatar, Iona. Their partnership began when Gunzō awakened the unresponsive I-401 under human custody; Iona revealed orders to serve under him, leading them to flee together.

Two years after securing the I-401, Gunzō and his crew accepted a Japanese government faction mission: deliver a prototype vibration warhead—capable of countering Fog technology—to the United States for mass production, aiming to empower humanity against the Fog blockade. During this voyage, Gunzō displayed keen tactical skill, identifying hidden enemy vessels and devising countermeasures against superior Fog warships, utilizing salvaged Fog weaponry and environmental traps.

Gunzō prioritized protecting allies regardless of origin. When the Fog battleships Haruna and Kirishima defected to protect the warhead's creator, Makie Osakabe, he intervened against Japanese forces targeting her. He declared his crew would operate by their own judgment beyond the delivery mission, defying external orders. His broader objective was establishing equal footing between humanity and the Fleet of Fog to end the blockade, a stance presented during negotiations with the Fog flagship Kongō.

In the sequel film *Ars Nova Cadenza*, Gunzō confronted the Fog Student Council, mental models enforcing the Fleet's planetary morality interpretation, and encountered his father, Shōzō Chihaya.