Cassios is a character from the anime and manga Saint Seiya, introduced as a formidable antagonist and rival to the protagonist, Seiya. A native of Greece, Cassios trained at the Sanctuary with the goal of becoming a legitimate Saint, specifically to earn the Bronze Cloth of Pegasus. His size and physical power are his most immediately apparent traits, as he stands over two meters tall with a muscular physique, white hair, and yellow eyes. A permanent mark of his defeat is his missing left ear, which he lost during his final battle against Seiya.

In terms of personality, Cassios initially presents as brutal, arrogant, and cruel. He believes that only a Greek is worthy of becoming a Saint, which fuels his hatred for the Japanese Seiya, whom he bullied throughout their shared training years. In combat, he is ruthless and has no qualms about attacking with the intent to kill or maim. However, beneath this harsh exterior lies a more complex individual. His defeat at the hands of Seiya humbles him to a degree, revealing a noble heart. The core of his personality is his deep, unrequited love for his master, Shaina of Ophiuchus, whom he respects profoundly. It is this love that ultimately redeems him and drives his most significant actions.

Cassios's primary motivation shifts over the course of the story. Initially, he is driven by a desire for glory and racial pride, seeking the Pegasus Cloth to prove the superiority of Greek warriors. After his defeat, his motivation becomes vengeance against Seiya, whom he blames for his humiliation and the death of his brother, Docrates, the Hydra Saint. Ultimately, his love for Shaina overrides his hatred. He realizes that Shaina cares for Seiya, and rather than see her suffer eternal grief, he chooses to act selflessly for her sake.

His role in the story is that of a foil and a tragic rival to Seiya. He first appears as the final obstacle in the tournament for the Pegasus Cloth, where his immense physical strength is pitted against Seiya's burgeoning Cosmo, the inner energy of a Saint. Despite his power, he is defeated because he has only developed his body and cannot effectively control his Cosmo. His most critical role comes during the Sanctuary arc. When the Gold Saint Aiolia of Leo is brainwashed by the Pope, Cassios learns that Aiolia will only return to normal after spilling the blood of his opponent, which is Seiya. To prevent the heartbroken Shaina from sacrificing herself to save Seiya, Cassios knocks her unconscious and goes to the Lion Temple in her place. He confronts Aiolia and allows himself to be killed, his sacrifice breaking the control over the Gold Saint and allowing Seiya to continue his mission.

Key relationships define his arc. His relationship with his master, Shaina, is the most important. He loves her deeply and holds her strength and skill in high regard. His rivalry with Seiya is born of jealousy, prejudice, and professional competition. Even in his final act, his intervention is not for Seiya's sake directly, but for Shaina's happiness. He also shares a connection with Aiolia; while not close, his sacrifice earns the Gold Saint's profound respect, as demonstrated when Aiolia covers his body with his own white cape.

Cassios undergoes a notable, albeit brief, development from a one-dimensional bully to a tragic hero. He starts as a proud and violent brute who relies on raw strength and despises weakness and foreigners. After losing to Seiya, he is consumed by shame and a thirst for revenge. In his final moments, he transcends these base motivations through an act of pure self-sacrifice born from love, proving his inherent nobility and earning a measure of peace and honor that he never achieved in life.

Regarding his abilities, Cassios is not an official Saint and does not possess a Cloth, wearing only reinforced leather protectors. His greatest asset is his colossal physical strength, which far exceeds that of a normal man, allowing him to crush opponents and handle heavy objects with ease. He knows basic martial arts techniques from his training but lacks the refined control over the Cosmo that defines true Saints. His techniques, sometimes named as Bloody Shoulder or Bloody Strangulation, are simple but powerful physical grappling and striking moves. In some non-canonical or expanded material, he is depicted as a Black Saint who develops a technique mimicking Seiya's Pegasus Ryu Sei Ken, but this falls outside his core portrayal. His most notable feat is not an offensive technique, but the ability to summon a faint white energy aura fueled by his powerful love for Shaina, which temporarily enhances his physical resilience.