It's a nice start, but more of like an outline. The hair, great detail! Height, eyes, and the fact she has braces, also a plus! But, there are other things that people have, right? Like, her skin color/race? Is she African, American, Canadian, Korean, etc? And if so, is she tan, pale, etc? Her weight is also important. Maybe you can say slim, and give a good estimate on her weight, and how body is built, i.e., an hourglass figure.
She doesn't trust anyone, yet she lets her guard down? That doesn't really make since. Maybe for family and uber closer friends, she trusts them a tad. But since she doesn't trust people, letting her guard down if you somewhat know her doesn't add up. Also, it says something happened to make her like this. Explain what, please. On what stuff is she clueless at? Because, if she's just uber smart and can be randomly, well, stupid, that makes no sense.
For dagger, what type? Like iron, celestial bronze, golden, silver, lead, etc. Also, you might want to add on how she is with the dagger. Novice, average, or expert with the dagger?
When you say she absorbs things, do you mean remembers them easily? Like, if someone says their middle name, in a year she'll remember it? Put more detail on that. Also, a child of Athena must have more brain power! I bet you could come up with other talents Penny would have that don't make her a Mary-Stu.
Her dad dead---try elaborating on how that makes her feel around the situation.
~Your fellow Awesomeness, Moo~