Helen of Troy

Helen of Troy (Ελένη in Ancient Greek) was the daughter of Zeus and Leda, queen of Sparta, wife of King Meneleus, and sister of Castor, Polydeuces, and Clytemnestra. Her abduction by Paris brought about the Trojan War. Helen was also said to be the most beautiful woman in the world, sometimes even more beautiful than Aphrodite.

History
The three Olympian goddesses Athena, Hera, and Aphrodite appeared to Paris and asked him to make a decision on who was the most beautiful goddess. As Paris was unable to decide, the goddess each offered him a gift in return if she chose them. Athena offered all the wisdom in the world as well as good swordsmanship and victory in war. Hera offered her power over the nations. Aphrodite offered to give her the most beautiful mortal in the world, Helen. After listening to that, Paris immediately chose Aphrodite, making Athena and Hera outraged, and 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 upon death by Zeus.
 * Some say she was later hanged by the Furies.

The Titan's Curse
Percy briefly mentions Helen when talking to Aphrodite about tragic love stories in a white limo at the Junkyard of the Gods 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 is picking out a weapon and picks Helen's knife, Katoptris which means looking glass, Annabeth reckons that's all it was ever used for, and she doubted it had ever seen war.