One of the most interesting clues towards Missy's identity is one of the first thing she ever says; in Deep Breath she not only shows great knowledge of the Doctor, but actually describes the Time Lord as her boyfriend. This limits the number of candidates down to a slight few. There's River Song, who actually married the Doctor, although that would be a little contrived at this point. And then there's the Rani, an evil Time Lord who appears time and again in theories of fans keen to see her return. But in terms of the current series, the most likely candidate to use that description is Clara. Sounds crazy but bear with us. In Deep Breath the Doctor makes it abundantly clear that he is not Clara's boyfriend. This can be taken simply as the show addressing the older Doctor and the its constant falling back on having will-they-won't-they esque relationships between him and his companions. However, with this contrasted literal minutes later with Missy's statement it seems to instead point towards something more. So, if Missy is Clara (and fair warning this paragraph is rampant speculation), how does that work? Is she one of the many versions we assumed all died to save him in The Name Of The Doctor? Or is she an older version (Jenna Coleman and Michelle Gomez do look vaguely similar), for some reason wanting revenge (we know the duo's relationship can splinter from this season alone)? Conclusion: Whatever Missy's identity is, odds are we've met her before. And while for them to be the same person would require some strange reasoning, who Missy is will likely play a lot into the plot of the finale, as well as whether Clara sticks around. Oh, yeah, we forgot to mention...
