Poseidon

Poseidon is the son of Kronos, god of the seas, Zeus and

Hades' brother Percy's and Tyson's father, and a character in the first film.

History
Poseidon was saved by Zeus from Kronos after being eaten by their father. He, Zeus, and Hades defeated Kronos and sent his remains in Tartarus.

Centuries later, Poseidon met and fell and love with Sally Jackson, a mortal. Together, Poseidon and Sally had a son, Perseus (nicknamed Percy). Poseidon loved his newborn son, but he began neglecting his duties as the sea god. Poseidon was forced to leave them behind. Zeus feared others would follow like his brother and had the other gods forbidden from contacting their mortal children, but Poseidon continued to guide Percy by telepaically talking to him.

when Percy was sixteen, Zeus accused Poseidon's son of stealing his Master Bolt. Angered, Poseidon threatened his brother with war should Zeus harm him, but Zeus was not intimidated and gave Percy until the summer solstace to return it.

Poseidon indirectly helped Percy on his quest by telepathically talking to him. He sensed his son in grave danger when he and his friends were in the Lotus Hotel and Casino and warned Percy to stop eating the lotus flower, which was keeping him and his friends from leaving.

Percy returned to the Empire State Building and returned the bolt that Luke Castellan stole, thus stopping the war between the gods. Zeus allowed Poseidon to talk with his son just once. Prior, Poseidon promised Percy that he might not be the father his son wanted but he will always be there for him.

the sea of Monsters
Poseidon is mentioned several times throughout the film. He sends his son and Percy's half-brother, Tyson, to Camp Half-Blood, where he meets his half-brother for the first time. Poseidon later sends a Hippocampus to Tyson, Percy and Annabeth so they can get on Luke's ship, the Princess Andromeda.