Amalie Stark is your extremely abnormal girl, she has always been, since her birth. Her father, if you didn't guess, is Tony Stark.
She is amazing at gymnastics, is scarily precise with a gun, has an IQ of 157 and has metal legs. 21 and living in New York, she is uncovered by SHIELD who want to put her brain and strength to good use. She meets the Avengers and one completely flips her life.
Based on Avengers Assemble and afterwards.