Poseidon is one of the 12 Olympians Gods, and after the split of the Earth, he dominated the Kaikai, while Atena dominated the Sekai (mainland), Zeus dominated Tenkai (Olympus) and Hades dominated the Meikai (Hell) and the Elysian (Paradise). Sent a flood to Sekai, and, had allowed only Noah and his family to survive the tragedy.
Like other olympians gods, Poseidon hates Atena and the way she governs the earth, leaving human sin and tolerating their faults.
Poseidon did not respect the frontier of the Empires of the Earth, therefore, he created armos for his seven generals, the scales.... thus, Poseidon almost defeated Atena, but she give armors for your saints too (the Cloths). Atlântida, where Poseidon ruled, sink in the sea, and the god was arrested in the North Pole.
After the first battle of Atena, other gods atacked the Sekai.
Poseidon returns in a battle for the Attica Peninsula, being defeated again, and sealed in your temple, in the Kaikai.
According Lost Canvas, Poseidon nearly Seraphina was awakened in the body, the sister of one of the descendants of those responsible for saving his soul at the North Pole, the Blue Warriors.
Poseidon is released by Kanon, Saga's brother, after he was trapped in a nearby prison. Poseidon, Athena recognizes that returned to life to fight Hades, and therefore decides attack Sekai with another purifier flood (but, the sinful humanity does he choose to kill everyone, leaving no survivor), possessing the body of Julian Solo, and begging Kanon wake up on your 16th birthday. Kanon, however, deceived Poseidon, letting him sleep in Solo's body, which is now handled by himself.
In the anime, Julian Sollo, to be a part of the sea god consciousness has Hilda with the Nibelung Ring, to weaken the army of Athena, but his plan fails.
Poseidon finally awakened in Julian after being hit by the arrow of Pegasus Seiya, and this is again imprisoned. Julian is not found by the Saints in time to be saved by them, and so, one of the servants of Poseidon, sacrifices himself to save him, for the sake of Julian himself and not in respect to his god.
Later, he, with Eris and Abel, revived Lucifer, which helps them to destroy the Earth.
Saint Seiya Omega
Poseidon returns in Julian's body to challenge Kouga and the other saints of the new generation.