I would recommend:
Anime: As clichéd as it sounds, Sailor Moon. That *was* the anime that my friends and I watched when I was 12-14. The awfulbad English version, no less!
Manga: Kodomo no Omocha. It stars kids younger than 12-14, but has enough humor and deals with enough deep issues to keep any preteen interested (probalby :-))
Novel: The Blue Sword by Robin McKinley. I read my hardcover copy so many times all the pages fell out. The main character Angharad Crewe (Harry as she is called for short) is about as tough as you can get.