Undercover 101

CALL OF DUTY AND MARVEL CROSSOVER
A 17-year-old is kidnapped by HYDRA during the 1920s because of her soon-to-be husband's business, she is injected with a serum that turns her into The Subject. Over the next 100 years, she is brainwashed and tortured while being sent on missions with the Winter Soldier to assassinate and kill. One day she escapes captivity after a mission goes sideways and ends up in the hands of British Intelligence, Task Force 141.

After running into them again after escaping captivity in Mexico and helping them escape the Mexcian Army, she decides to join the Task Force. She comes up with a believable backstory and works alongside the Los Vaqueros and 141 for a good 2 years until Brock Rumlow and his band of misfits come banging at the gate of the Los Vaqueros Base ready to capture the Subject and bring her back to HYDRA.