Saint Paul's is really Missy's TARDISHow else would the 3W institute fit inside?
If Missy is you-know-who, then who is Seb?Who is Missy's non-evil-seeming assistant? He's friendly, but friendly doesn't mean non-evil (Hell, Missy didn't prove she was the villain until the "say something nice so I can feel better about disintegrating you" bit.) So who is he?
- He's a computer program, an avatar designed to act friendly and inviting by the Cybermen or Missy. His friendly demeanor won't falter for a second if and when the time comes for him to start doing bad things.
- Seb is likely a part of the Master's personality, but whether he is an AI or actually also the Master himself (if split-regeneration is even possible) remains to be seen. After all, two minds are better than one and while Missy is off distracting the Doctor and UNIT, he could be putting the final stages of the plan in order, of which the Doctor won't discover until it's nearly too late. Which is the Master's plan.