Hermione is bookish and clever, with a memory to match. She has a penchant for inventing new spells, though thinking on her feet isn't exactly her best quality. Oh she can; she's just not as quick as she'd like to be.
She is easily exasperated by Ron, and often horrified by Harry - or rather, the danger Harry seems to find himself in. She tries to be their voice of reason, but even she has an occasional bad streak in her - to break the rules for the greater good.