After civil war Bucky went to Wakanda to safely remove his HYDRA brainwashing and regain his freedom. Steve arranged so that someone trusted would come with him and help him through his journey. It so happened to become Y/n, a trusted and reliable Avenger that had been in the team for a long time. They also so happened to have had a past with HYDRA themselves and were good friends with both T’Challa and Shuri. So it was the best choice… in more ways than expected.