Medea

Medea was the princess of Colchis, the land that held the Golden Fleece in the time of the original Jason. When the hero was sent on a quest to claim the fleece, Medea fell deeply in love with him. Her father agreed to grant Jason his request, but only if he accomplished impossible and deadly tasks. With the help of Medea's magic however, Jason was able to steal the Fleece from the king and escape.

Jason at first agreed to take Medea with him and marry her upon returning to Greece, which he did. But soon afterward he left her in order to marry Princess Creusa instead following the death of King Pelias. Heartbroken, Medea wrought a terrible revenge by poisoning Jason's Creusa, and the people of Corinth in anger murdered Medea's two children from her marriage to Jason. Helios, Medea's grandfather, then sent a flying chariot driven by golden sun dragons to take her away to Athens.

At Athens, Medea was married to King Aegeus and had another son, Medus. One day, Theseus came to Athens. Medea recognized him as Aegeus's son and worried that he would become king instead of Medus. So she sent Theseus to capture the Marathonian Bull, an emblem of Cretan power. Theseus returned victorious, however, and sacrificed the bull. Medea then tried to poison him. At the last second, Aegeus recognized the sandals, shield, and sword, and knocked the poisoned wine cup from his hands. He banished Medea, and she supposedly fled to Asia.

The Sea of Monsters
In The Sea of Monsters, Annabeth mentions Medea's Sunscreen.

The Titan's Curse
In The Titan's Curse, Dionysus mentions her while explaining to Percy why he hates demigods, citing all the people who heroes have betrayed.

The Demigod Files
In a diagram of Annabeth's Camp Trunk, there is Medea's SPF 50000 sunscreen, which is strange considering she left hers at home in The Sea of Monsters.

The Lost Hero
Jason, Piper and Leo come to her mall, where she introduces herself only as the Princess of Colchis. She begins to show the demigods her stock of magic objects and potions, but becomes angry when she hears Jason's name, as he is named after the hero that betrayed her. It quickly becomes evident to Piper that Medea is up to no good. Piper attempts to snap them out of it with her own charmspeak and the stories of the horrible things Medea commited in the past. Eventually, the two come to their senses, and they blow up the mall. Medea shouts that she will not be abandoned again as she was by Jason and almost manages to gain Piper's sympathy. They choose to leave her however when they realize that Medea was only trying to buy more time to destroy them. She is an agent of Gaia, and the one who Hera had warned them about. Jason and Leo fall under the effects of Medea's charmspeak and she uses the influence to turn them against each other. Piper isn't affected, since she can also charmspeak.

Trivia

 * Medea used a diamond instead of a drachma for an Iris message in The Lost Hero. It is unknown how she did this (it is possible she used magic).