Nikolaus Geist, also known as Doctor List, was a scientist who worked for S.H.I.E.L.D., but was known to be one HYDRA's top prime figureheads. At the time of the HYDRA Uprising, he broke ties from S.H.I.E.L.D. around the time of the Battle at the Triskelion. As the right-hand man of Baron Wolfgang von Strucker's branch of HYDRA, Geist had done a lot of experiments with Loki's Scepter and also on the enhanced people.
In the aftermath of Daniel Whitehall's untimely death, Geist's actions inadvertently led to S.H.I.E.L.D.'s discovery of the exact position of Strucker's Sokovian base. He was assumed dead when Iron Man blasted the HYDRA Doctor with a repulsor beam when the Avengers, alerted by S.H.I.E.L.D., attacked Strucker's base. After fleeing Sokovia, Nikolaus Geist retreated to his personal headquarters in Berlin, Germany.