Theseus

Theseus (Θησέας in Ancient Greek) was the son of Poseidon (and Aegeus) and the princess Aethra and founder-king of Athens.

History
Theseus, after King Minos kidnapped some Athenian boys and girls to sacrifice to the Minotaur, decided to descend into The Labyrinth to save them.

He was aided by the old inventor Daedalus and the mortal princess Ariadne, and with their help able to navigate the Labyrinth and defeat the Minotaur.

The Titan's Curse
In The Titan's Curse, Dionysus stops Percy and Blackjack while they were following Thalia, Grover, Zoe, and Bianca. He says that you can never trust heroes. Theseus left Ariadne all alone. So, Dionysus married her.

The Battle of the Labyrinth
In The Battle of the Labyrinth, he appeared while Nico was trying to summon Bianca. Percy thought that Theseus was older and bigger until he saw his ghost, which surprised him.

The Last Olympian
In the Last Olympian, he is briefly mentioned by Paul and Nico while they are discussing how Percy will send a signal to his mom and Paul if he defeats Kronos.

Paul suggests, "Like Theseus. He was supposed to raise white sails when he came home to Athens," to which Nico replies, "Except he forgot, and his father jumped off the palace roof in despair. But other than that, it was a great idea."