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, 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, a magical item she created. She was later mentioned by Circe, when she was naming all the powerful women in history.

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, M's, 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. As Jason and Leo only want to keep shopping, Piper notices that Medea is charmspeaking them into buying whatever she wants them to. Piper attempts to snap them out of it with her own charmspeak and the stories of the horrible things Medea committed in the past. As Medea continues to charmspeak them to the point that they are barely listening to Piper as Medea tells them to chop each other into pieces and she would bring them back to life even stronger (the same way she killed Pelias). As Leo and Jason pull out their weapons, Piper gets Medea to admit that she can see the future and knew that Leo would rise up against Gaea, so she warned Gaea from the Underworld. This event set in motion the awakening of Gaea and the death of Leo's mother. As a reward, Medea was set free of the Underworld and allowed to return to the land of the living, as long as her mall remained underground so Gaia could keep watch on anyone that entered. Eventually, the two come to their senses, and they blow up the mall after Festus appears, who helps Leo and Jason to defeat the two sun dragons which attacked them by order of Medea. 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 Gaea, and the one who Hera had warned them about. As they fly away with Coach Hedge and the storm spirits (caged in the bottom floor of Medea's mall), they look back to make sure the she wasn't chasing them.

Appearance
She is described as a beautiful, thin woman with long red painted-nails, long dark hair and a gorgeous face. She looks like a model and wears an elegant black dress with diamonds jewelery. After rising from the Underworld, she seems to have a faint glow around her and when she gets angry, her skin becomes slightly transparent causing her skull to become somewhat visible.

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).
 * She was known for potions.
 * She can charmspeak.
 * She still keeps the sun dragons her grandfather Helios had given to her when she escaped from Corinth.