"Anthea"—or whatever her real name is—is playing around on her cell phone again as her employer, Mycroft Holmes, talks to his little brother and his little brother's totally-not-boyfriend. She's too involved in texting her friend—another PA for Mycroft—to really listen.
She enjoys her job. She's had it for nearly twenty years now. For someone who had such an important position in the British government, Mycroft didn't appear to have too many requirements to become his PA, and she was able to get hired not long after finishing high school. Maybe he figured someone young and without a college education wouldn't lead to his downfall. She isn't sure, but work is work.
It's a simple job—dangerous, of course, but simple. Make calls, get information, and manage the surveillance cameras. Naturally, there is a lot of secrecy involved. One little slip up and they could all end up dead. Not just Anthea and Mycroft, but England itself! There were a lot of rules in place to help keep the entire delicate operation running, but above all:
Never use your real name.
Anthea's gone through a lot of aliases over the years—Courtney Kingsley, Rosie McIntosh, Jenny Cougar—but "Anthea Morstan" is her favorite. No one knows her real name (apart from Mycroft, of course, but even he normally refers to her as Anthea). She's used more fake names than she can count. At least a hundred. Probably more.
"We must be going," Sherlock says, rather abruptly. Anthea smirks to herself, wondering what her employer did to piss off his brother this time.
"And a good day to you, too," Mycroft says. The sarcasm is so thick you could cut it with a knife.
Sherlock and John begin to walk out, passing Anthea in her place on the couch, still texting her friend.
"Okay, I have to ask," John says, stopping beside her. "What's your real name?"
"I can't tell you," she replies, not taking her eyes off her phone.
"I won't tell anyone."
"I know you won't."
"So what is your real name?" he asks again.
She begins to tell him to sod off again, but then pauses to think. In the eighteen years she's been working for Mycroft, he's never called her by her birth name. Not since the day she was hired. Had anyone called her by her real name? Her friends were all involved in the government—she didn't know their real names and they didn't know hers. She didn't have any family. No one left who didn't know her as "Anthea."
Come to think of it… If her name isn't "Anthea," then what is?
Finally, she answers him.
"I don't know. It seems I've forgotten it."