She is the most experienced S.H.I.E.L.D agent, well besides Nick Fury, Nick is the one that found her when she first discovered her powers. She is more experienced than Natasha, and that says a lot. She was the one that trained Natasha when she first came to S.H.I.E.L.D. Her name is Sibyl, she usually is quiet, but she is very stubborn, hard-headed, will talk back, sarcastic and she will break a person's finger if she is annoyed by them. When she is on a mission, she is a whole different person, her attitude changes and she becomes a killer, someone who will not hold back at all. So, when Nick tells the Avengers about Sibyl and that she will be part of the team, who knows, maybe it will be good for her, and them.