Wikipedia
Demaratus was a king of Sparta from 515 until 491 BC, 15th of the Eurypontid line, successor to his father Ariston. As king, he is known chiefly for his opposition to the other, co-ruling Spartan king, Cleomenes I. He later migrated to Achaemenid Persia where he was given asylum and land, and fought on the Persian side during the Second Persian invasion of Greece.
For the Corinthian of the 7th century, see Demaratus of Corinth
Demaratus was a Corinthian prominent amongst the pro-Macedonians and connected by hospitality with the family of Philip of Macedon. It was through the mediation of Demaratus that Alexander returned home from Illyria, as a consequence of the quarrel between himself and his father after the marriage of Philip with Cleopatra. He was almost certainly present in Aegae when Philip was assassinated in 336 BC, and in 334 he accompanied Alexander on the Asiatic expedition as one of his hetairoi. He is said to have wept with joy to see Alexander seated on Darius' throne at Susa. Demaratus died shortly before the Indian campaign; his remains were sent back to Corinth with appropriate honors.