"Missy," says the Doctor.
"Hmm?" says Missy abstractedly, picking out a few notes on her piano.
"I was wondering."
"I'm sure you were."
"When you met me in this regeneration of yours, you said 'I couldn't very well keep calling myself the Master.'" He looks at her with that burning stare. "Why not?"
She giggles. "Oh, darling."
"I mean, I get that you're doing the performative femininity thing, fine. But in that case, why not just Mistress?"
"Even you can't possibly be such an innocent," she says with a parody of a sultry pout.
"But that's exactly what I mean," he says, unconsciously rubbing the tip of his nose, where she once kissed him while pretending to be a droid. "The innuendo and ambiguity, the combined implications of dependence and power and desire—it all seems very you, actually."
"Stop analyzing me," she says. "You're not in one of your classes now."
He gives her another hard look.
"If you must know," she says, "I did use 'Mistress' for a while. But every time someone said it, I felt like I was talking to that awful little dog thing you used to have." She adopts a squeaky mechanical voice. "'Affirmative, Mistress. Negative, Mistress. Adequate supply available in triple container, Mistress.' Do you know your prissy little girlfriend gave that thing the Coronet of Rassilon?"
So very many things he doesn't want to talk about. He laughs and sighs and lets the argument drop.
She knows what he was hoping she'd say: I wanted to distance myself from my past, I wanted to be different, I wanted to be better.
She'll never tell.