Helen of Troy

Helen is the daughter of Zeus and Leda, daughter of King Tyndareus, queen of Sparta, wife of Meneleus and sister of Castor, Polydeuces and Clytemnestra. Her abduction by Paris brought about the Trojan War. Helen is also said to be the most beautiful woman in the world.

History
When the three goddesses Athena, Hera and Aphrodite appeared to Paris and asked him to make a decision on who was the most beautiful. As Paris was unable to decide, the goddess each offered him a gift in return if he chose them. Athena offered all the wisdom in the world as well as good swordsmanship. Hera offered him power over the nations. Aphrodite offered to give him the most beautiful woman in the world, Helen. After listening to that, Paris immediately chose Helen.

The Titan's Curse
Percy briefly mentions about Helen when talking to Aphrodite about tragic love stories in a white limo at the Junkyard of the Gods.