Casshern is the central figure of his story, a powerful being who awakens in a desolate, decaying world with no memory of his past. He exists as a living contradiction, simultaneously the world's greatest destroyer and its potential salvation. His background reveals that he was once the strongest warrior serving Braiking Boss, the ruler of the robot army. In this role, he was ordered to assassinate a girl named Luna, who was considered a source of life and salvation for humanity. Casshern succeeded in his mission, but the act of killing Luna triggered a cataclysmic event known as the Ruin, a slow, corrosive death that spread across the world, affecting both robots and humans. Following this event, Casshern vanished, only to reappear hundreds of years later with no recollection of who he was or the devastation he had caused.

Personality-wise, Casshern is defined by a profound and pervasive sorrow. Upon learning of his role in causing the Ruin, he is consumed by guilt and a deep sense of despair. He does not relish his own power and, in fact, despises fighting. His demeanor is quiet and withdrawn, often carrying the immense weight of his unseen past. He is frequently driven by a desire to atone for his sins, which manifests as a passive acceptance of the hatred and violence directed toward him by other robots who blame him for their suffering. He shows little regard for his own wellbeing, a direct result of his overwhelming guilt. At the same time, he possesses a gentle side, especially in his interactions with the innocent and kind-hearted, such as the young girl Ringo.

His primary motivation throughout the story is twofold: to understand the truth of his past and to find a way to atone for it. He seeks out Luna, believing that finding her might hold the key to understanding why the world is dying and why he alone seems immune to the Ruin. Beneath this quest for answers is a powerful death wish; his guilt is so overwhelming that he genuinely wants to die, unable to forgive himself for his actions, even though he cannot fully remember them.

In the story, Casshern serves as the protagonist and the focal point of the world's despair and hope. He is a person of mass destruction, a living weapon of immense power, yet his journey is not one of conquest but of introspection and redemption. He wanders a post-apocalyptic landscape, constantly under attack by robots who believe that by killing and consuming him, they can gain his immunity to the Ruin and achieve immortality. His role is to confront the themes of life, death, and mortality that the story explores, acting as a grim reaper figure who forces both others and himself to contemplate the meaning of existence.

Key relationships shape his journey significantly. His most significant connection is with Lyuze, a female robot who initially pursues him for revenge, believing he killed her sister, Liza. Despite her hatred, she eventually becomes his closest companion and ally, with their bond deepening into a complex and emotionally charged partnership. Ringo, a young robot girl who retains her innocence, offers Casshern unconditional kindness and faith, seeing him not as a monster but as a friend. Her pure belief in him provides a stark contrast to the world's hatred and serves as a source of emotional support. Friender is a loyal robotic dog who accompanies Casshern, often intervening to protect him or others, especially when Casshern loses control of his own power. Dio and Leda are his former colleagues, warriors created alongside him with similar specifications; Dio in particular harbors an intense rivalry with Casshern, his existence defined by the desire to surpass and defeat him. Braiking Boss is his former master who ordered the assassination of Luna, the true architect of the world's ruin who now observes Casshern's journey from the shadows.

Across the narrative, Casshern undergoes significant development. He begins as an amnesiac wanderer, passive and accepting of the world's hatred, believing he deserves to die. As he travels and forms bonds, particularly with Lyuze and Ringo, he begins to move beyond his self-loathing and accept his past. His development culminates in a philosophical evolution where he stops seeking death and instead embraces a new purpose. He understands that the fear of death is what gives life its value, and he chooses to become a symbol of death itself, a guardian of mortality to ensure that those who achieve eternal life do not become complacent.

Casshern possesses several notable abilities that set him apart in his world. He is an invincible hero, a cybernetic being with superhuman strength, speed, and reflexes that make him the most powerful fighter in existence. His most defining ability is a powerful and painful regenerative healing factor; he can recover from almost any wound, which contributes to his status as an immortal. When pushed too far, either by a threat or extreme emotional duress, he enters a violent berserker state where he loses control and mercilessly destroys everything in his path. In this state, his eyes glow and his facemask closes, signaling the unleashing of his full, terrifying power. Despite his reluctance to fight, once engaged, he is a graceful and devastating force, capable of tearing apart entire groups of opponents with his bare hands.