Heracles Algethi was a silver saint in XX Century, and the strongest silver saint, in tribute a Heracles. He was sent to spy Aiolia (with Dio and Sirius), because he iis the brother of the biggest traitor of the Sanctuary (Aiolos). After Aiolia allows Seiya get away and survives, Algethi, Sirius and Dio attacks Seiya on their own, and playing a game: who will kill Seiya first?
Only Dio and Algethi attacks Seiya, because before Sirius attack, Seiya uses the Saggitarius Cloth to kill the 3 silver saints.
In the Hades Arc, Algethi goes to fight against Shun to doesn't disturbs Shion scheme.