Speaking to Doctor Who Unleashed, Davies said: “So that’s goodbye to Millie Gibson, except it’s not! You’d have to be mad to say goodbye to talent and a character like that!”
He added: “This is a pause. I genuinely felt Ruby’s story paused there. She couldn’t get all that information about her family, all that emotional overload, and run off in the TARDIS. It pauses there. She’s coming back.
“A new companion is coming in, but you’ll see the three of them together. Three people in this TARDIS fighting evil. There’s good stuff to come. There are really crucial stories for Ruby to come, and her family – the story of that family hasn’t finished yet – that will all make sense when you see it. So more to come.”
I will say, though, that I interpreted one of Ruby’s references to “my real mum” as a reference to Carla. I think it might have been in episode seven, but I forget the details.