Fifteen-year-old Luna Kozuki is the daughter of a scientist aiding the human resistance against the Black King's robot army. She shares a childhood bond with Tetsuya Azuma, who transforms into the cyborg warrior Casshan. Luna actively fights alongside the resistance, wielding a specialized MF Gun. Her vital contributions include delivering weapon plans to human forces and masterminding revolts within enemy strongholds like Castle Promise, a robot factory where she rallies enslaved workers.

Suffering betrayal by a fellow prisoner, Luna is captured and slated for public execution to crush rebel morale, only to be rescued by Casshan. Later, on a critical mission to deliver weapon designs to Admiral Rudolph, her transport helicopter is shot down; the pilot sacrifices himself for her escape from robot scouts, leading to her reunion with Casshan and his robotic companion Friender.

Throughout the conflict, Luna anchors Casshan emotionally as he grapples with his lost humanity and physical vulnerability, remaining by his side despite his detachment. She participates in pivotal operations, including the assault on a bridge crucial to the robot army, witnessing Casshan's internal struggle and renewed resolve. In the war's final phase, Luna attempts to comfort Casshan after a spectral warning from his mother about the Black King's counterattack, persisting in her support despite his initial rebuff. She survives to witness Casshan's final, fatal battle against the Black King. The Black King's defeat triggers a fail-safe, shutting down all robots, securing humanity's victory and Luna's place in the liberated future.