Nereus appears to have been the personification of the sea in its calm and placid moods, and was, after Poseidon, the most important of the sea-deities.
He is represented as a kind and benevolent old man, possessing the gift of prophecy, and presiding more particularly over the Ægean Sea, of which he was considered to be the protecting spirit. There he dwelt with his wife Doris and their fifty blooming daughters, the Nereides, beneath the waves in a beautiful grotto-palace, and was ever ready to assist distressed mariners in the hour of danger.
From: Berens, E.M. The Myths and Legends of Ancient Greece and Rome. New York: Maynard, Merril, & Co., 1880. Text in the public domain.