Anyway, I will name my heroine... Serendipity.
(The mean girls will call her Pity.)
Her magical power will, fittingly, be ridiculous luck, and she will, of course, be powerless to control it—it will usually kick in when she's done something amazingly embarrassing due to her astounding clumsiness.
(Because, of course, there has to be some Bella Swan in her—but in giving her that power, I'm giving her EVEN LESS AGENCY than Bella. Why I've decided that I want my imaginary YA novel to be even more annoying than Twilight, I don't know...)