Falcon & The Winter Soldier: 10 Characters Who MUST Appear
3. Johann Fennhoff
In the first season of Agent Carter, Johann Fennhoff's machinations and hypnotic abilities caused quite a problem for Peggy Carter and the SSR. By the end of the season, Fennhoff - in a revenge plot against Howard Stark - nearly succeeded in using a brainwashed Stark to unleash a chemical weapon on thousands before he was stopped and brought to justice.
Last we saw of him, he was cellmates with Arnim Zola, who immediately offered him the opportunity to begin collaborating together. Though we haven't seen Fennhoff since, the existence of things like the Faustus method used by HYDRA members like Daniel Whitehall and Sunil Bakshi on Agents of S.H.I.E.L.D. and the brainwashing technology used on Bucky makes it all too clear that Fennhoff and Zola's collaborations paid off in spades for the evil organization.
Like Zola, should the series ever step back in time to explore some of Bucky's past, an appearance by Fennhoff - even briefly - would be a fun way of tying the MCU's history together, especially with James D'Arcy's Edwin Jarvis having set a precedent through his cameo in Avengers: Endgame.