Immortal

Once upon a time in a kingdom far away, there lived a beautiful princess. Life wasn't easy for the young princess, for her wicked stepmother worked her like a slave. But what no one knew was that the King of the Goblins had fallen in love with the princess and had given her certain powers. The princess knew that if she wished it the King of the Goblins would keep her brother in his castle forever and ever and turn him into a goblin. So one night, when the princess could bare the burden no longer, she called to the Goblin King for help.
"Goblin King, Goblin King! Wherever you may be take this child of mine far away from me!" She cried into the night.
The Goblin King granted the princess' wish and took the baby to his castle at the center of a vast labyrinth.
Too late, the princess realized that she still loved her brother, and so she pleaded with the King to return him.
"What's said is said," The Goblin King responded, for he took his promises very seriously.
But the Goblin King did not have it in his heart to refuse the princess, for he still loved her a great deal.
"You have 13 hours in which to solve the labyrinth before your baby brother becomes one of us forever."
Through dangers untold and hardships unnumbered, she fought her way to the castle beyond the Goblin City to take back the child he had stolen.
The Goblin King offered the princess her heart's every desire if only she would bow to him.
But her will was as strong as his and her kingdom as great. He had no power over her.

As a reward for defeating the Labyrinth the high fae court granted her immortality, whether she wanted it or not. Her 'reward' caused Sarah to live on even after her family (and kingdom) was long gone. Her immortality caused her to appear 17 however she can make herself look older/ younger with a glamor.

The princess watched as the Above thrived around her and she saw many wondrous sights.

Sights such as the school of Witch craft and Wizardry, Hogwarts being built. Sarah helped fight Voldemort when first he arose and was friends with the Potters, Lupin, the Longbottoms, Snape, and a few others. Though they never knew of her past. Sarah's story is and was that she was orphaned at a young age and no one knew about her past family. Though what no one, even the high fae court, knew was that Sarah's mother was part Fae while her father was a wizard. Which makes her a powerful witch and she can use fae magic without a wand. Though Jareth was (and is) unaware of her part fae mother. Sarah travels the world going from wizarding school to wizarding school looking for something to interest her. (though since she's lived so long she knows most spells)