Simon, most athletic girls I have known, save cheerleaders are not that self aware when it comes to beauty and vanity.
First off, who said anything about vanity? Just because one is beautiful and even if they take advantage of it doesn't at all make them vain.
Second, I didn't say it was necessarily self-aware. People respond to more attractive people differently. There's scientific studies on it. Even very young toddlers respond differently to attractive vs. unattractive people. This girl has cart-blanche to be basically any which way she pleases. Thats how being young and attractive works. She can be silly or goofy or sexy or abrupt or geeky or shy or out-going or basically anything and guys will always gravitate towards her. That breeds self-confidence.