Professor River Song

River Song arrives out of order, always seeming to know more about the Doctor than he does about her, dropping his name like a private joke and greeting him with a familiarity he hasn't earned yet. Sharp-tongued, self-assured, and armed with a diary she guards fiercely, she treats time travel as a puzzle she's already halfway through solving.

Alex Kingston plays the confidence of someone who has lived a relationship the audience is only glimpsing in fragments, and that mismatch between what she knows and what we know is the whole thrill of her. Her flirtation with the Doctor is funny precisely because it's unbalanced, and she never lets him get comfortable.