Nereus was born in the first generation of the Greek cosmos, the son of Pontus, the primordial sea, and Gaia, the primordial earth. In Hesiod's Theogony, the great poem of divine genealogy composed around 700 BCE, Nereus appears as the eldest child of this union, a god who inherited the calm and the depth of his father's waters and the steadfastness of his mother's soil. He was not a storm god like Poseidon, the Olympian who ruled the sea with his trident and his temper. Nereus was the sea at peace, the gentle swell that carries ships to harbour, the clear water that reveals the sand beneath. The Greeks called him Geron Halios, the Old Man of the Sea, and they imagined him as a white-bearded figure emerging from the waves, his body covered in seaweed, his eyes the colour of the deep water, and his voice the sound of waves breaking on a distant shore.

He married Doris, one of the three thousand Oceanids, the daughters of Oceanus and Tethys, the primordial river-gods whose waters encircled the world. Doris was a nymph of the fish and the coral, and she bore Nereus fifty daughters, the Nereids, each one more beautiful than the last. The most famous of these nymphs was Thetis, whose beauty was so great that both Zeus, the king of the gods, and Poseidon, the god of the sea, desired her. But a prophecy warned that Thetis would bear a son greater than his father, and the gods, fearing the fate that had befallen Uranus and Cronus, married her instead to Peleus, a mortal king of Thessaly. From that union came Achilles, the hero whose wrath shaped the Trojan War. Other Nereids included Galatea, the beloved of the Cyclops Polyphemus, and Amphitrite, who became the wife of Poseidon and the queen of the sea.

Nereus's defining characteristic was his truthfulness. Unlike other gods who lied to mortals for sport or for revenge, Nereus always told the truth, but he would only speak to those who proved themselves worthy. The test was a wrestling match. Nereus was a shape-shifter of extraordinary power, able to transform his body into any creature that lived in the sea or on the land. When a mortal seized him and demanded answers, Nereus would change form again and again, becoming a lion with claws of coral, a serpent with scales of pearl, a leopard with spots of shadow, a tree with roots of kelp, a torrent of water that slipped through the fingers, and a flame that burned without heat. Only the mortal who held on through every transformation, who refused to release their grip no matter what horror or wonder appeared in their arms, would win the right to ask a question and receive a true answer.

The most famous wrestling match was with Heracles, the son of Zeus and the mortal woman Alcmene, the hero who was forced to complete twelve labours as penance for killing his wife and children in a fit of madness sent by Hera, the queen of Olympus. The eleventh labour required Heracles to fetch the golden apples from the garden of the Hesperides, the nymphs who tended the tree of golden fruit at the western edge of the world, but no mortal knew the way. Heracles travelled to the shore of the Aegean and found Nereus emerging from a cave at dawn, the old god's beard dripping with seawater and his eyes still heavy with sleep. Heracles seized him, and Nereus began to transform. For three days and three nights, the hero wrestled the god through every shape the sea could produce, until Nereus, exhausted and impressed, returned to his true form and told Heracles the way to the garden. He also told him that only Atlas, the Titan condemned to hold up the heavens, could pluck the apples without arousing the serpent Ladon that guarded the tree.

Nereus was worshipped at Gythium, the port city of Sparta in Laconia, where a sanctuary dedicated to him stood beside the harbour. Sailors offered sacrifices to him before embarking on voyages, and fishermen prayed to him for calm seas and abundant catches. In art, he was depicted as a wise old man, sometimes riding a sea-monster, sometimes holding a trident or a staff of coral, always surrounded by his daughters who danced in the waves at his feet. He was not a god of terror or power but of knowledge and endurance, the elder of the sea who had seen the world before the gods were born and who would remain when the last ship had sailed.