I'm considering a female British student on her gap year, who accidentally frees the Lady Of The Lake of Arthurian Legend from some really crappy prison like a sea shell or something, and the Lady of the Lake develops a crush on the girl and gives her the sword Excalibur, which also gives her the right of True Queen of England. She completely ignores this and focuses on using the indestructable sword to fight crime.
She'd have absolutely no training or experience, and be kind of a loser. Just some random everyday gal who just so happens to have a really cool sword.