Actarus, also known as Duke Fleed, is the protagonist of Grendizer U. He is the crown prince of the planet Euphor, who arrives on Earth as a refugee after his homeworld is destroyed by the forces of the Vegan Empire. To conceal his identity on Earth, he adopts the name Daisuke Umon.

By nature, Actarus is a noble, generous, and deeply altruistic individual who embodies the ideals of a Starker knight. He is consistently kind and selfless, often placing the well-being of others before his own. His forgiving and compassionate nature earns him the admiration of his allies but also leaves him vulnerable to betrayal and emotional pain. A defining aspect of his personality is the immense guilt he carries over the destruction of his home planet. In a fit of rage following the murder of his parents, Actarus piloted the giant robot Grendizer but lost control, causing it to go berserk and devastate Euphor, inadvertently killing civilians and even attacking the ship of his fiancée, Vegalia. This trauma makes him deeply reluctant to fight, as he fears losing control again and causing further catastrophe.

Actarus's primary motivation is atonement for his past sins and the protection of his new home, Earth, from suffering the same fate as Euphor. He is determined to prevent the Vegan Empire from obtaining Grendizer and using its power for conquest. This sense of duty is complicated by his deep-seated fear of his own power, leading him to initially consider destroying Grendizer and himself to ensure it can never be used again. Throughout the story, his role is that of Earth's secret defender, reluctantly piloting Grendizer to fight off the Vegan invaders while hiding his true origins from the general public.

Key relationships heavily influence his journey. His love for Princess Vegalia is a central emotional driver, and he is haunted by the belief that he killed her during his rampage, a misunderstanding that fuels his desire to clear his name. He shares a deep, lifelong bond with Teronna, a princess of Vega and a fellow Starker knight, who was his best friend and former sister-in-law. Their relationship becomes strained when she witnesses his destructive失控, but she remains conflicted due to their shared history. On Earth, his most crucial relationship is with Alcor, who rescues and befriends him. Alcor's unwavering trust is instrumental in helping Actarus regain his resolve; he gives Actarus a ring that doubles as a remote explosive, promising to use it to kill him if he ever loses control of Grendizer again.

As the series progresses, Actarus develops from a man paralyzed by guilt and fear into a more resolute hero. The trust placed in him by Alcor and his desire to protect his new friends help him find the strength to pilot Grendizer again, not for revenge, but for defense. He learns to confront his past and his enemies, even showing mercy to those who have wronged him, such as taking his former friend Sargas prisoner instead of killing him.

In terms of abilities, as a Starker knight, Actarus possesses telekinetic powers that can enhance his speed and strength. He is also a highly skilled pilot and swordsman, trained from childhood. Notably, his blood is compatible with that of humans, a fact that allows him to save Alcor's life through a transfusion. His true power, however, lies in his extraordinary willpower and noble spirit, which are often the deciding factors in his battles. He pilots the colossal robot Grendizer, a machine that can only be operated by a Starker knight and which is prone to entering a destructive berserker mode when Actarus is consumed by rage or a desire for vengeance.