Ariadne in Theseus: The Minotaur and the Labyrinth
Poor Princess Ariadne really gets the bum end of the deal in this myth. Even though she makes it possible for Theseus to escape the Labyrinth, the hero breaks his promise of marriage and deserts her on an island. Ariadne reminds us a lot of Medea who helped Jason get the Golden Fleece, only to be later deserted by him. Of course, Ariadne doesn't go crazy and kill her kids afterward, so maybe she's a bit more sympathetic.
There are a lot of variations on the story of what happens to Ariadne after Theseus ditches her, but most agree that she ended up married to Dionysus, the god of wine. The two had lots of babies and ended up living on Mt. Olympus. Some say that the constellation Corona is named after the crown that Dionysus placed on her head. So, yay, not everything in Greek mythology is tragic.