Dio is a pivotal figure in the narrative, serving as a rival to the protagonist. He was created as a humanoid robot, built to be physically identical to Casshern. Along with Casshern and a robot named Leda, Dio was part of a trio produced by the scientist Oji during the rule of the tyrant Braiking Boss. This trio was designed with the unique capability to reproduce, a feature not found in other robots. Originally, Dio and Casshern functioned as elite killing machines for Braiking Boss's empire. However, a fateful mission to assassinate a being named Luna, who was seen as a savior, became the defining event in Dio's existence. While Casshern succeeded in killing Luna, an act that triggered a worldwide cataclysm known as the Ruin, Dio was left behind, an event that fueled his deep-seated resentment.

In personality, Dio is defined by a cold, harsh, and blunt demeanor. He is driven by a powerful, obsessive jealousy towards Casshern. Before the assassination, Casshern was always considered the superior warrior, which created a significant inferiority complex in Dio. After the Ruin began and Casshern disappeared, Dio’s envy hardened into an all-consuming desire for revenge. His primary motivation is singular and unwavering: to be the one who defeats Casshern in combat. Above all else, he wants to prove his own strength by surpassing his former colleague. To that end, he breaks one of the horns on his helmet to visually separate himself from Casshern, as he despises being mistaken for his rival.

Within the story, Dio acts as a major antagonist and a dark mirror to Casshern. While Casshern wanders the ruined world seeking answers and redemption, Dio actively pursues him from a position of power. He declares his intention to kill Casshern and, influenced by Leda, attempts to build a robot army, proclaiming himself as the leader of a new empire. His role is to challenge the protagonist not just physically, but ideologically, embodying a path of hatred and obsession as a response to a world without hope. His relationship with Leda is complex and central to his journey. Leda acts as his manipulator and companion, encouraging his ambitions and caring for him, but Dio is ultimately driven by his own desire for battle rather than her dreams of eternal life and power. Despite Leda's manipulation, Dio’s final wish reveals a genuine, if unexpressed, love for her, asking Casshern to save her.

Dio undergoes significant development, transforming from an envious soldier into a warrior who finds purpose in his obsession. Early in his pursuit, his uncontrolled rage weakens him, leading to defeat. He learns that his hatred must be channeled into a passionate, life-affirming desire to fight for its own sake. In his final confrontation with Casshern, he experiences what he considers a "true life" in the heat of battle—a feeling he says eternal life could never replicate. This allows him to achieve his goal and land a fatal blow on the immortal Casshern, securing the victory he always wanted. With his purpose fulfilled, he allows the Ruin to claim him and dies satisfied.

As a combatant, Dio possesses formidable abilities. Having been built with the same specifications as Casshern, he shares many of his physical traits, including superb reflexes, incredible speed, and immense strength that allows him to easily crush or pierce other robots. He is a master of hand-to-hand combat, using swift punches and kicks, and his agility can make his movements appear as teleportation. However, a critical distinction exists between him and his rival. Unlike Casshern, who is immortal and possesses instantaneous regenerative abilities, Dio is mortal and can be wounded. While he can heal over time, he does not share Casshern’s invulnerability, making him a more fragile, albeit equally dangerous, opponent.