Cassios is the apprentice and saint in training under Ophiuchus Shaina. He's the younger brother of Docrates. During the fight for the Pegasus cloth, Seiya cut off one of his ears and he hated Seiya ever since.

Spoiler

Casios has great respect and affection toward Shaina, however he realize that Shaina has affection toward Seiya. Motivated by this fact, he takes Seiya's place and dies during the fight with Leo Aiolia, in order to wake Aiolia from the control of Saga's Genrō Maō Ken.