Col. Castle is a high-ranking officer within the Central Intelligence Agency, serving as the immediate superior to Agent Max Reed. He is a man who operates with a calm and authoritative demeanor, often appearing detached from the more chaotic events that unfold around him. Unlike the overtly monstrous agents of the Chronos Corporation, Castle maintains a professional, bureaucratic facade, but this veneer of government service masks a deep-seated corruption. He is secretly in league with the Chronos Corporation, the powerful and sinister organization seeking to recover the Guyver unit and achieve world domination.

Throughout the story, Castle is motivated by his alliance with Chronos, working to ensure the corporation's objectives are met from within the intelligence community. He oversees an operation to extract the scientist Dr. Tetsu Segawa and retrieve the Guyver unit, intending to hand the powerful artifact directly over to Chronos. When the mission goes wrong due to the actions of the Zoanoid Lisker, Castle shifts his focus to damage control. His primary role becomes obstructing his own subordinate, the more principled and suspicious Agent Max Reed, from investigating Chronos and uncovering the truth. To this end, he assigns other agents to follow Reed around the clock, effectively placing his own man under surveillance.

Castle's key relationships are defined by his duplicity. His professional connection with Max Reed is one of false camaraderie, as he attempts to dismiss Reed's suspicions with dry, evasive remarks, such as when he speculates on a scientist's late-night activities by saying, "Some people will do anything to land a fish. Even tell a fish story". In contrast, his relationship with the Zoanoid Striker evolves into something more pragmatic. After most of the Zoanoids and their leader, Fulton Balcus, are defeated, Castle approaches Striker with a simple proposition, indicating that he is a survivor who seeks to acquire new assets for his own purposes. This is demonstrated in the film's final scene, where Castle, standing with Striker, coolly removes a cigarette from the Zoanoid's mouth and offers him a job, suggesting a new partnership is formed.

As a character, Col. Castle shows little to no development in terms of a moral arc. He begins and ends as a calculating and untrustworthy figure, leveraging his government position for corporate gain. His story concludes not with a comeuppance but with him simply moving on to the next phase of his plan, having survived the chaos that claimed many others. In terms of notable abilities, Col. Castle possesses no superhuman powers or combat skills. His primary strength lies in his authority, his strategic mind, and his ability to manipulate the system. He is a man who uses power and position rather than physical force to achieve his goals, standing in stark contrast to the bio-boosted Guyver and the monstrous Zoanoids he associates with.