Last week, we saw The Doctor create a functional immortal out of Maisie Williams' Ashildr in “The Girl Who Died.” Just as we'd predicted, immortality took its toll on her psyche, and “The Woman Who Lived” re-introduced us to the character who now goes by the name of “Me.” Eager to travel with him, but ultimately denied by The Doctor, Me / Ashildr has decided to become a sworn defender of the Earth. The only problem is, she's defending the world from The Doctor and the consequences of his actions.
The funny thing is, this could have been prevented if The Doctor had given in to Ashildr's request to travel with him. Echoing, as well as referencing, his previous adventures with Captain Jack Harkness, we see a human who's been given the lifespan of a god. Much like his previous companion, Ashildr wants to travel with The Doctor because she needs the “shared human experience” that he mentioned as a requirement for an immortal. Which reveals a lot about why The Doctor rejects immortals as his companions. In fact, The Doctor will never take on an immortal as a companion--no matter how much he likes them--because he needs to stay grounded.
While it hurts him when his companions leave him, he ultimately understands that a tacit requirement of being a Time Lord with a vastly extended life span is that he needs someone who reminds him of the fragility and responsibility his abilities require. We've seen time and time again that even someone like The Doctor can lose themselves in a situation, especially in “The Runaway Bride” and “The Waters Of Mars,” only to realize there's an issue after it's too late. In both cases, the one thing that was missing was a full-time companion. Without a familiar, mortal foil to keep him grounded in his reasoning, The Doctor can get a bit brash and angry with his foes, and unless he's careful that might end up happening again.
With Clara's impending departure hanging over this entire season, her absence from this episode only punctuated this eventuality further. When she does return at the end of the episode, proudly proclaiming she isn't going anywhere, the look of sadness on The Doctor's face says it all: it's only a matter of time before she does go. One of the following two things could certainly happen toward the end of the season: Clara's departure is so upsetting that The Doctor possibly does take on Ashildr / Me as a companion, or The Doctor's adventure with River Song during this year's Christmas special will cause him to beg her, once again, to travel with him on a full-time basis.
With the final moment of the episode showing Ashildr / Me in the background of a selfie Clara took with one of her students, how or when she'll come into play again remains a mystery. While their last conversation ended on a somewhat friendly note, the time span between “The Woman Who Lived” and the present era would be enough time for her to forge a hatred for The Doctor. One thing is clear through all of the ambiguity: she's out there, and if she's not careful, her intentions toward The Doctor and his companion could very easily turn sour.
Doctor Who airs on BBC America, Saturdays at 9 p.m. EST.