This is revealed throughout the story through various clues and confirmations, including:
* Percy's abilities: Percy possesses control over water and can breathe underwater, traits directly linked to Poseidon.
* The prophecy: The prophecy about "the child of the big three" who will bring about a great war hints at the involvement of one of the Big Three gods (Zeus, Poseidon, and Hades).
* Luke's revelation: Luke Castellan, a major antagonist, confirms Percy's parentage and explains the prophecy's significance.
* The Olympian council: Percy is formally introduced to the Olympian council as Poseidon's son.
While Percy doesn't have a traditional "father figure" in the story, Poseidon's identity as his father significantly influences his journey and the overall plot of *The Lightning Thief*.