From the moment she first appeared, sneakily tucked away at the end of the series 9 trailer, and asked that question of The Doctor ("what took you so long, old man?), the internet has been going crazy as to who exactly Maisie Williams will be playing. After all, she's the big new guest star, arriving as one of the heroes of television's biggest show, and that teasing glimpse certainly suggested there was going to be a lot more to a character. It was assumed that she'd be someone major from his past, with a number of characters from both new and classic Who suggested. She is defintely River Song! No, she's Susan, The Doctor's granddaughter. Actually, she's Jenny, The Doctor's daughter. Nope, you're wrong, clearly it's Romana, the Time Lady from the 70s... I think you'll find it's quite obviously Clara, because why not?! Well, we were all wrong, as Moffat has confirmed that she won't be someone from his past, and will be a new character. This has led to some re-theorising, such as that instead of actually being Susan, she'll instead be Susan's mother, or Susan's daughter, or Susan's dog given human form. Basically most of the theories still centre around her being someone very much linked to The Doctor's past, albeit in a roundabout way - i.e. being the daughter or such like of a character we've already met. Unless, of course, Moffat is lying.