She was brought to HYDRA as a child. Raised in Siberia, with the rest of The Winter Soldier’s. Including the first, James Buchanan Barnes. The serum effected her differently due to her age, and other factors. When Zemo goes to Siberia to kill them all, she senses him a mile away, and hides. Bucky’s first instinct when he gets there is to check the cryo freeze chambers for her. She’s not there. And she feels the grief he has for the one kid in the facility who still had her soul.