First episode: A Study in Pink
Seen also in: The Empty Hearse
Occupation: Assistant to Mycroft Holmes
Show history: She is sitting in the black limousine John is ordered into by a mysterious villain (which we now know is Mycroft). Rather than give John the time of day, she texts away on her phone, barely looking up, and only giving cryptic answers to John's questions, along with a condescending smile. She does give a name when John asks, but "Anthea" seems to be a name she pulls out of the air, as she readily admits that it's not actually hers.
Eventually (and predictably), John tries to hit on her. She barely notices, once again texting on her phone, and when she does, her only reply is raised eyebrows, an amused smile, and a sassy "Bye!" When we see her again at the end of the same night, John says hello to her; she says hello back, obviously having been too preoccupied with more important things to remember John or how they met. John makes his exit, probably vowing never to try that again.
When Sherlock "resurrects," as it were, Anthea is the one to assist in making him Sherlock Holmes again, including bringing in the Belstaff coat we all know and love.
My take on her: Personally, I love Anthea. Lisa McAllister is obviously having a lot of fun playing the character, and, for a role that easily could have been for a faceless extra, giving Mycroft's assistant a name and a personality was a smart decision, and definite comedic gold. For having such little screen time, the character makes her mark, which is why Moffat and Gatiss brought McAllister back in "The Empty Hearse" after four years of being off the show. In regards to the character's role within the show, how fascinating would it be to be Anthea, the insider on the life of Mycroft Holmes, and a key part of all his political and business dealings? No wonder Anthea takes no notice of "ordinary" people like John; she's probably seen the rise and fall of the country, and all the power plays possible within the British government, in that very limo! While Anthea doesn't play a large role within the series, the actress is so good, I would be delighted to see Anthea back in Season 4, and any season onward. Imagine the possibilities for that role, what great insight she could give us on Mycroft's life, something we don't often get to see. Of course, we can't take away the mystery and vague menace of his position as the British government, but perhaps she could provide a glimpse into the life of what is obviously a very powerful, very lonely man.
And, I wonder if we'll ever find out Anthea's real name? Probably not; I suppose that's one mystery that's intriguing, but doesn't necessarily need to be solved!