Phantom (Avengers)

Phantom is on Shield's monitors. She is no one special, at first glance, but Shield found some rather interesting footage of her. To them, she is a phantom. She appears in places she shouldn't be and next second she's gone. They try to find a pattern in her strange appearances, but find nothing. They don't know her name, but have been calling her Phantom. Years after the project of finding her started, it was forgotten. Phantom disappeared completely.
Except she didn't. She was found, five years after the sudden stop of showing up on cameras in random places. She was found hardly alive on the doorstep of the Avengers tower.
After the wars with both Loki and Ultron, the avengers are a bit bored, living in the tower, waiting for something interesting to happen, for the world to need them again. When Tony Stark stumbles across the mysterious Phantom, the world is just about ready to need them, especially when they find out who Phantom really is.