Actarus, originally known as Duke Fleed, is the Prince of the annihilated planet Fleed. Fleeing his homeworld's invasion by Yaban forces, he commandeers their experimental giant robot Gattaiger to escape to Earth. Under the alias Daisuke Umon, he resides near Mount Fuji under scientist Genzo Umon's guardianship and works at the Makiba ranch.

When Yaban attacks Earth, Actarus unveils his identity and pilots the Gattaiger to defend his adopted world. The robot wields faster-than-light flight, projectile weapons including Spider Spin buzzsaw and Needle Shower missiles, and beam attacks like Space Thunder and Thunder Focus. During the conflict, he reunites with his fiancée Telonna, but she falls in battle against Yaban commander Blaki. Her death prompts Actarus to depart Earth aboard Gattaiger, abandoning his terrestrial life.

Actarus maintains a reserved, pacifistic demeanor in daily life, contrasting with his ferocity as Gattaiger's pilot. His survival of Fleed's genocide fuels his motivations—first driving his escape, then his resolve to protect Earth. This character's design and narrative role became the prototype for Duke Fleed in *UFO Robot Grendizer*, where core elements like Fleedian royalty, the Makiba family, and antagonists such as Blaki were expanded.