Who is Poseidon from the novel, The King Must Die?

Poseidon is the god that Theseus thinks himself to be the son of. While the reader never sees Poseidon we do listen to the conversations of Poseidon that Theseus partakes in. We know what Theseus tells us of him and he is the only other character in the story that is a constant from the beginning till the end of the story. At the front of the tale Theseus explains that he feels a kinship to the Horse King who will later be sacrificed to Poseidon. He believes that Poseidon is his father and that the Horse King is his brother since Poseidon can make himself into the shape of either a horse or a man.