Rabi, sworn protector of Apolon’s monarch, dedicates himself to safeguarding the royal bloodline after the Dazaan Army assassinates the king. Amid Apolon’s devastation, he smuggles the infant Prince Takeshi to Earth, entrusting him to the Blue Sky Orphanage with a cryptic pledge to reunite when the boy reaches sixteen.

When the fated year arrives, Rabi returns to unveil Takeshi’s destiny as Apolon’s heir and Earth’s defender through the planet’s ancient guardian, the colossal mecha Dai Apolon. He reveals the Dazaan’s theft of the Energy Heart—Apolon’s vital power core—and mentors the prince in awakening the “Key Energy” embedded within his chest, a royal birthright granting mastery over the combining robot.

As both strategist and guide, Rabi deciphers the conflict’s roots in General Dazaan’s conquests while orchestrating missions to reclaim the Energy Heart and avenge Apolon’s fall. His alien physiology—marked by intricate facial patterns and an ethereal bearing—reflects his extraterrestrial lineage, grounding his authority as the prince’s last tether to their ravaged homeworld.