Avengers

Y/N L/N used to be friends with Steve and Bucky when they were kids. You got taken by HYDRA not long before Bucky did. He doesn't know what happened. He thinks you're dead. He blames himself. You're a shapeshifter. Only Bucky, HYDRA, and SHIELD know. SHIELD eventually finds you. You get sent to live with the Avengers. You refuse to shift to human. Fury thinks they'll help.