She was Miranda Priestly's personal assistant and it was the name the main character (Anne Hathaway) was often called after by mistake (as she wasn't appreciated enough for her boss to bother to learn her real name).
This is someone who works very hard and is always doing a million things for their boss and their work is never appreciated.
B: Wow, that sucks!
A: Yeah, I'm like my office's Emily!
Me: Oh yeah i talked with emily for 5 hours last night.
Guy 1: Dude i am so jealous.
Guy2: no thats just emily
Emily: "Whoa!! Check it out dude!! Look what I made!"
Hero: "Uh.. OH! WOW! That's awesome, Em!! How did you do that?!!"
Emily: "I'm not telling!!"
Guy2: Oh, let me introduce you to Emily!