During the fight for the Pegasus cloth, Seiya cut off one of his ears and he hated Seiya ever since.
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.
Cassios' punch block by Seiya KotZ (Netflix Series)
KotZ (Netflix Series) Cassios wearing the Black Armor & it shocking