Poseidon

Poseidon, also known as Earthshaker, Stormbringer, and the Father of Horses, is the god of the sea, earthquakes, and the creator of horses. He and Athena are rivals after Athena won a city (Athens) that he wanted. He is also, along with his brothers Zeus and Hades, one of The Big Three, one of whose children is the Prophecy Child, who will decide the fate of Olympus. That is why the brothers took an oath after WWII to never have half-blood children again, which he and Zeus broke, even though Nico and Bianca di Angelo are children of Hades.

History
Poseidon spent his childhood in his father's belly, along with the rest of his brothers and sisters. When Metis and Zeus tricked Kronos into vomiting them up, Poseidon joined their cause and battle against the Titans, along with his siblings Hades, Demeter, Hestia, and Hera.

After they won the war against the Titans, the three brothers; Zeus, Poseidon, and Hades drew lots. Zeus and Poseidon, had drew the sea and sky. Hades got the Underworld as this was the only realm left after his brothers.

Poseidon built a palace in the sea, and replaced Oceanus as the lord over the sea, and married one of his daughters, Amphitrite. She became Queen of the sea, and they ruled together. They had a son, and named him Triton.

Poseidon is the father of many heroes, and sired Theseus.

Rivalry with Athena
For many eons, Athena and Poseidon have had rivalry between them. It all started when they competed for being the patron of the city Athens. The leader of the city told the two gods to make a gift for the newly constructed city. Poseidon made a useless salt-water spring, while Athena, gifted them the Olive Tree. Seeing that the Olive tree was much better than a useless Salt-Water Spring, The leader of the newly constructed city named it after Athena (Hence the name Athens) and made Athena a temple known as the Parthenon, and also made her the Patron of the city. Also, Athena once caught Poseidon with his girlfriend Medusa (who was once a pretty maiden). Furious at Poseidon and Medusa for doing such disgusting acts in her temple, she turned Medusa and her sisters into the "Three Gorgons". Medusa's sister's were also turned because they helped Medusa get into the temple. The competition for Athens and the terrible acts Poseidon committed to in Athena's temple are probably the basis for why Athena dislikes Percy.

The series
Poseidon dated Sally Jackson, a beautiful mortal woman that he called 'queen among women' and that he hadn't found a woman like her in a thousand years. They vacationed together at Montauk, and conceived Percy. After a while, Poseidon left, telling Sally to send their baby to Camp Half-Blood when he was old enough.

Poseidon later visited Percy when he was a baby, and Percy always remembered the warmth of his father's smile, unaware his mother thought that they had never met.

When Percy made it to Camp Half-Blood, he didn't know who his father was. Poseidon knew that Percy was his son, but had yet to tell him.

After Percy survived the hellhound's attack in The Lightning Thief, Poseidon claimed him. This caused discord among the gods of Olympus, as this was proof that Poseidon had not honored the pact he had made with his two brothers. After Zeus' lightning bolt was discovered to be missing, Zeus suspects Poseidon used Percy to steal it, and demands he return it by the Summer Solstice, and Poseidon in turn wants an apology for being suspected on that date.

A quest is given to Poseidon's son Percy to find the missing lightning bolt and return it in time in order to stop a war amonst the gods. He succeeded, and Poseidon got to meet his son formerly. Poseidon was very sorry for Percy, because being a hero means a tragic end for most heroes. Percy told his father he wasn't sorry for being who he was, and Poseidon told him that he was a true son of the Sea god.

In The Sea of Monsters, Poseidon does not appear, but he is said to have sent his Cyclops son Tyson to the same school as Percy, in order to get them to know each other, send the Hippocami to Percy and his friends,

In The Titan's Curse, Poseidon talks to Percy about Luke, and about his prophecy. Percy assures his father he isn't afraid. Poseidon vouches for Percy and Bessie in the end, saving Bessie's life.

Fighting the Titans
In The Last Olympian, when Percy sees Poseidon, he looks aged and weak, unlike the man he once knew. Poseidon was busy fighting the Titan of the sea, Oceanus, who was constantly attacking his kingdom. Poseidon had been fighting so hard, his strength had been deteriorated. However, Percy was able to convince Poseidon to come up from his kingdom and help bring down Typhon, which greatly aided the gods  in their fight to save Olympus. He was very proud of Percy, for fighting so strong, after the battle was over.

Family
He is the father of the Demigod, Percy Jackson by Sally Jackson, and Tyson the Cyclops. He is also a full brother to Zeus, Hades, Hera, Demeter, Hestia, and a son of Kronos.

His mother is Rhea and he (like his other siblings) has a half-brother, Chiron, the Centaur who helps run Camp Half-Blood.

He is married to Amphitrite, who seems to hate Percy at a first glance, due to all his affairs with mortals. Triton is his immortal son and next heir to be the Sea God.

He loves his sons Percy and Tyson, he also seems not to have a problem with Percy's relationship with Annabeth, and is glad Percy has found someone to love.

It is hinted in the end of The Last Olympian that he has more unclaimed children, though it's unclear whether or not he's kidding, as he both winks as though he's joking, and then reminds Percy to "know which ones can land", obviously talking about the future demigods that will appear, though whether or not he's talking about his own children. Poseidon also has an immortal family in his underwater palace.

Personality
Poseidon is a very kind god. He has a quick temper, like Percy, but is a honest man. He cares deeply for Percy, and calls him his 'favorite son'. At first Poseidon didn't know what to think about Percy, but grew to love him.

Appearance
He has black hair, neatly trimmed beard, and sea-green eyes. He wears leather sandals, khaki Bermuda shorts, and a shirt with coconuts and parrots on it. He sometimes is seen wearing Birkenstocks. He used to wear a battered cap decorated with fishing lures. It said NEPTUNE'S LUCKY FISHING HAT.

Powers
Poseidon is an extremely powerful god, capable of seriously wounding even Typhon as shown in the climax of the Second Olympian War. Although originally believed only to be second to Zeus, in the climatic battle it is shown he is likely more powerful than even him when he quickly seriously wounds Typhon, making him bleed a giant waterfall of golden blood, something all of his siblings were unable to accomplish in a week.

Children

 * Tyson
 * Percy
 * Triton

Film
In the upcoming film The Lightning Thief, Poseidon will be played by Kevin McKidd.