She based the character of Hermione off of her 11-year-old self.
When asked if she based Harry Potter characters off people she knew, she responded "some of them are, but I have to be extremely careful what I say about this. Mostly, real people inspire a character, but once they are inside your head they start turning into something quite different. Professor Snape and Gilderoy Lockhart both started as exaggerated versions of people I've met, but became rather different once I got them on the page. Hermione is a bit like me when I was 11, though much cleverer."