Dioscuri were ancient twin gods who helped out humanity as guides in their voyages, such as being the manifestation of St. Elmo's fire in ships. But as humanity developed, they were merged with the Olympian pantheon as minor Divine Spirits, stripping Castor out of his divinity and reducing Pollux to being just a demigoddess. As Servants, the two manifest as singular Servant with two separate bodies with two different classes. Pollux is a Saber, and Castor is an Avenger of hate.