Nereus from a Vase painting
A Titan who, with Doris, fathered the Nereids. Together, he lived with his family in the Aegean Sea. He was a shapeshifter with the power of prophecy and he would aid heroes, such as Heracles, who managed to catch him even as he changed shapes. Thus Nereus and Proteus ("first") seem to be two manifestations of the god of the sea prior to being supplanted by Poseidon, when the latter's brother Zeus overthrew Cronus.
Nereus from the Pergamon Zeus Altar Gigantomachy
Nereus was known for his truthfulness and virtue:
But Pontos, the great sea, was father of truthful Nereus who tells no lies, eldest of his sons. They call him the Old Gentleman because he is trustworthy, and gentle, and never forgetful of what is right, but the thoughts of his mind are mild and righteous. — Hesiod, Theogony 233
Karl Kerenyi, The Gods of the Greeks
Robert Graves, The Greek Myths