Helen of Troy

Helen of Troy was a Greek demigod, a daughter of Zeus, and the mortal, Leda. She was the queen of Sparta, wife of King Meneleus, sister of Castor, Pollux, and Clytemnestra and she bore a daughter named Hermione. Her abduction by Paris brought about the Trojan War. Helen was said to be the most beautiful woman in the world, sometimes even more beautiful than the goddess Aphrodite.

History
Zeus, in the form of swan, was chased by an eagle. He sought refuge in Leda, wife of King Tyndareus of Sparta, and she gained her affection so they mated, and Helen was born as an egg, originally. The three Olympian goddesses Athena, Hera, and Aphrodite appeared to Paris and asked him to make a decision on who was the best goddess. 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 and victory in war. Hera offered him power over Asia Minor. Aphrodite offered to give him the most beautiful mortal woman in the world, Helen. After listening to that, Paris immediately chose Aphrodite, making Athena and Hera outraged, which made them Paris' enemies. This was eventually orchestrated by Eris as Paris claiming Helen for his wife even though she was already married to King Menelaus. This action ended up starting the Trojan War. When Troy fell thanks to the Greeks using the Trojan Horse, Paris was killed and Helen was reclaimed.

The different outcomes of Helen have varied for each story:
 * Some say she was brought up to Mount Olympus by Zeus, after her death.
 * Some say she was later hanged by the Furies.
 * Some say she never made it to Troy, but that Hera, in order to take revenge from Aphrodite, made Hermes kidnap Helen and bring her to Egypt. While Helen was in Egypt, Hera made a fake Helen. After the war was over, Menelaus lost his ship and landed in Egypt. There the fake Helen disappeared and Menelaus and Helen tried to find a way to escape, without the king Theoklimenous taking notice on their escape. Eventually they came up with a plan. Menelaus was going to pretend to be a lost sailor, and they would say that Menelaus was dead. Then Theoklimenous gave the real Menelaus a ship and food so he and Helen managed to escape.

The Titan's Curse
Percy Jackson briefly mentions Helen when talking to Aphrodite about tragic love stories in a white limo at the Junkyard of the Gods when she is with Ares somewhere in Colorado. Aphrodite sees these tragic love stories as if they were soap operas. She also says she hasn't seen such a tragic love story for centuries, probably referring to Helen and Paris.

The Lost Hero
She is mentioned when Piper McLean is picking out a weapon and picks Helen's knife, Katoptris, which means looking glass, Annabeth Chase reckons that is all it was ever used for, and she doubts it ever saw war. Later at Aeolus' Palace, Jason Grace mistakes the wind nymph Mellie for a ghost, and Piper covers for him, saying he mistook Mellie for Helen of Troy. It seemed a little over the top, but Mellie appeared to buy it. Later still, Aphrodite appears to Piper in a dream to tell her that she is more powerful than she thinks, referring to Helen as having been one of her "favored ones", inferring that it was men's love for Helen that started the Trojan War.