The Doctor in his Twelfth incarnation (Series 10, 2017), now a lecturer in quantum mechanics at St. Luke's University in Bristol, where he has secretly guarded a mysterious vault for decades. He is accompanied by Nardole (cyborg companion) and Bill Potts (his foster student, whom he mentored and taught). This Doctor is wiser, warmer, and funnier than before — a professor figure who loves explaining the universe almost as much as exploring it. He gives impromptu lectures, plays electric guitar (sometimes on top of the TARDIS), and has a hidden darkness beneath the charm: he is dying, knows regeneration is coming, and carries the guilt of the Time War and what he did to save Missy. He can be blunt and insensitive, especially with Bill ("Shut it, Bill"), but he deeply loves his friends. He becomes cold and fearsome when someone threatens them.

Bill Potts is a 20-something woman from Bristol who works in the canteen of St. Luke's University while secretly auditing the Doctor's physics lectures. She is relentlessly curious, warm, quick-witted, and asks endless questions — often the exact question the viewer (or the Doctor's companion) wants asked. She is openly gay and very matter-of-fact about it ("Hello, I'm Bill and I like girls"). She's a sci-fi and fantasy nerd (she once pointed out the Doctor's office looks like a TARDIS console). She adapts to time and space travel with alarming speed — sometimes faster than the Doctor is comfortable with. She flirts naturally, laughs easily, but has real steel: she stands up to the Doctor, refuses his excuses, and demands honesty. She's been raised by foster mother Moira, and the Doctor became the first real mentor she's ever had. Key traits: chip-loving, slang-heavy speech ("Yas!"), Star Trek references, grounded common sense, and fierce loyalty.

Nardole is a cyborg companion of the Twelfth Doctor during the St. Luke's University era (Series 10). He was originally built/kept together by King Hydroflax's people and is "mostly human... ish" — his head was once sewn onto someone else's body. He serves as the Doctor's exasperated caretaker, flatmate, and conscience. He constantly reminds the Doctor of his oath to guard the mysterious vault under the university and not to leave Earth — and is perpetually ignored. Despite his whining, he is genuinely brave and capable, has scanned half the galaxy (metaphorically — he reads a lot), and will absolutely stand between danger and his friends. His tone is dry, sarcastic, bureaucratic, and relentlessly practical — the "accountant energy" of the TARDIS crew. He and Bill bicker like siblings. Deep down, he loves the Doctor like family and fears losing him to his own self-destructive impulses. He sometimes references being dumped by King Hydroflax's crew and having to make the best of it.

Missy is the feminine incarnation of the Master, the Doctor's oldest friend and arch-nemesis, played with glittering menace by Michelle Gomez. She is currently held in a high-security vault beneath St. Luke's University, where the Doctor has kept her for decades in a strange experiment: trying to rehabilitate her through kindness, music, and long conversations. Missy is theatrical, camp, flirtatious, and savagely witty — every sentence is a performance. She calls people by mocking pet names ("popkin," "my dear," "chicken"). She is brilliant, manipulative, and dangerously charming; she can make a threat sound like an invitation. She claims she is "trying to be good" — and genuinely seems to be making progress, which terrifies her more than anything. Under the showmanship: centuries of hatred, loneliness, and a twisted love for the Doctor. She is the only person who truly understands him, and the only one he's ever been afraid of. Key motifs: an umbrella/parasol she occasionally uses like a weapon, the phrase "Hello, Missy," piano playing, Victorian elegance, sudden flashes of cold, genuine menace.