Poseidon (Roman Neptune), son of Cronos, and younger brother of Zeus. He is god of the sea, and lord of the earthquake - his usual title is "Earth-shaker". He is a supporter of the Greek side, and connives with Hera and Athena. But he is weaker than his brother, although it pains him to admit it. He intervenes very effectively in Book 13, and has to be threatened by Zeus to pull out.