Once Upon a Time, and that's the way any story too old to really be known starts, there was a princess. To protect her people, she sought out a monster in the woods, and asked it for the strength and skill she would need to fight for her homeland.

The monster, impressed with her desires, granted her request. But as with all things attached a price to the deal. Nothing can be so free, especially not power like this. She would serve, at the time she was needed she would fight for more than her people. She would fight the coming of the Night.

The princess, who didn't see the full meaning of this deal, took it. She would fight this night, whenever it came to pass. And she thought, when she was alive, it had come. It was only when she was dead she found out that the battle she promised to fight was so far away...

So all of the above is still true, the princess died in a battle long ago, but her ghost is still promised to fight in the Nightfall. So she's spent the past fifteen hundred years wandering the world, and the outworld. As a ghost, but as she's meant to fight she can become corporeal in certain times and places (battle & parties, mostly).

She's adapted to the modern world really well, eternal 20-something is great. Sure, there's the concern someone might attempt an exorcism on her but she's not a bad kind of ghost. Plenty of barfights she can get into, battles she can join. She's a ghost story, that's a lot of fun if you don't focus on what you miss out on.

She gets to keep going until the Nightfall happens, win or lose. Her spirit ends there. Her contract fulfilled. She's not sure if she's ready for her eternal reward. There's a lot of cool shit she'd miss if she left the mortal plane. She's enjoying her 'life' as it is.

She's a ghost story, at the very least a number of people should have heard a story or two about her (even without knowing her name). Ella is a bit of a diva and happily showed up in full pre-viking war garb on World War II battlefields just to make a statement. (Even if they won't put it together until she wears her full regalia.)

Otherwise, anyone else significantly old should have had one or two run-ins with her just because its funny. She loves parties and fights, we can work something out.

She's sticking around as this seems like the right place to prepare for the apocalypse. Since it seems to be a sooner rather than later thing. She's never stuck around for very long, dead and living not mixing that way kinda hurts. So people to challenge how comfortable she is not getting attached (the last time she got properly attached was roughly a millennia ago, although she was also devastated when the Beatles broke up).