Gerold Rösler was a German agent working in Constantinople in 1914.
Rösler worked closely with Turkish prison official Colonel Çelebi. They were concerned about an artefact that had been discovered by Maurice Bernard and stolen by British agents.
When the Seventh Doctor was taken to the prison, he managed to convince Rösler and Çelebi of the artefact's danger. They assisted him in recovering it. (PROSE: Ante Bellum)