She's a middle-aged female time traveler who talks in a sassy seductive tone, is constantly keeping an eye on the companion throughout the series, and wears some form of optical device at all times. So basically she's Madame Kovarian, except an actually decent villain. On top of being ominous, she's actually funny at times and gets her own companion in the form of Karen, who she has some witty "ugh dumb humans" banter with. In the story Grand Theft Cosmos, she turns a palace guard into a sleeper agent who will become hypnotized upon a certain trigger phrase; the phrase she decides upon is Elvis Presley because he wouldn't be born for another 37 years and thus no one would be likely to say it. That's how ya do villain humour right there, not retarded-arse John Simm and Missy being all "LOOK AT ME, IM MAD AND INSANE HAHAHAHAHAHAHAHAHAHA!"