She chose it deliberately, so that every time someone said her name it would remind her of why she has to be careful. And because, while there's small likelihood of the real Emma looking for her in a downtown homeless shelter -- even if she sends someone to ask around, this is the last pseudonym any of them would expect.
There's nothing to connect Emma No-Last-Name-Asked-Or-Given with a vanished former NYU student named Beth Lehrer. Her hair is darker -- luck rather than planning; she can't afford to keep tinting it blonde -- and shorter; she gave some thought to wearing glasses, but realized she has no idea how to find a pair with nonprescription lenses. Any more than she had any idea how to disappear into suburbia and create a plausible new identity, or survive in hiding in the sewers -- two options that came to mind before she hit on this one.
Because she obviously couldn't go home again.
That one phone call to her family's house was a mistake.
"BETH?" She had to hold the stolen cellphone away from her ear at her sister's shriek. "Beth, what happened, where are you? Are you all right?"
"Laura, I'm --"
"We've been trying to reach you for months, Mom and Dad are going crazy, where have you been? Where are you calling from?"
"I'm okay," and she hopes fervently it's not a lie. "I'm in the city. It's--"
"You just disappeared! Mom's had the police looking for you, and people from your job were calling our house trying to reach you --"
Beth jerked, snapped the phone shut, flung it aside, and started to run.
That was months ago, and she hasn't dared call back since. There are ways to trace calls, and she doesn't know enough about them to try to circumvent them. And that's not even counting the possible ways that her mind still shies away from contemplating, the ones that shouldn't be possible but somehow are anyway.
The thing is: she hasn't gotten away from it. Away from the Council, yes; away from Wolfram & Hart, yes. (She hopes.) But not away from that world. Once you know what's out there, you can't forget.
Which means that when the bedraggled man in the bread line starts muttering loudly to himself about vampires, she can't shrug it off as the crazed ramblings of a wino. So she does the next best thing: ducks her head to hide behind her hair, averts her eyes, and hands out the next piece of bread.