The 7th Avenger

Emily was abandoned when she was two years old and is found by a S.H.I.E.L.D agent who takes her in and trains her to be an agent. But she also has powers that one day could become very useful. On her 13th birthday Loki comes to take back the tessaract. So they call all the avengers. Even though she isn't an avenger at first she earns the title down the road of what will happen.