In classical mythology, Tethys was the Titaness of the Ocean, the sister/wife of Okeanos, and the mother of the 50 Oceanids (ocean nymphs) and of all the Rivers. She was also the guardian of the goddess Hera before her marriage to Zeus.
Not much is known about her, as she never enters battle and seldom speaks. She is not loyal to Kronos, her youngest brother and king: by the end of Saint Seiya: Episode G, she is known to have betrayed him. Together with Okeanos and Mnemosyne, she became a follower of Gaia and Pontos.