Is a graphic novel cheating? If it is I don't care. Because it's this lady, without a doubt.
355, from Brian K. Vaughan's Y: The Last Man
355 is the baddest babe around. She is tough as nails and smart and sassy and she has a horrifying fucked up past and she isn't afraid of anything and she's sexy as HELL and I could give specifics but if you ain't read these comics it would be better to just read them, and if you have read them then you know what I'm talking about.
God I love her, I love her so much. She's cooler than anyone. She's my spirit guide.
And an honorable mention from a "real" book, because you can't have Will without her:
Lyra, from Philip Pullman's His Dark Materials trilogy
Lyra is also a total badass and smart and sassy and not afraid of anything, except when she is, and she's capable of such wonderful tenderness also, and oh god how does Pullman get little girls so well, I just, I can't.
Lyra will always remind me deeply of my best friend, who is also blond and small and fierce and loves cats.