Chief prosecutor in the town of Visaria, Erik Ernst was a very fair and idealistic young man who was romantically involved with Elsa Frankenstein, having great respect for her father Ludwig Frankenstein. When Ygor arrived in Visaria with the Frankenstein Monster, Erik was given the unenviable task of prosecuting the creature (who was by this point in his unnatural life almost entirely brain-dead), after he had murdered two innocent townspeople.
Erik summoned Ludwig Frankenstein to the courthouse during the trial, hoping that his work with mental illnesses would make him qualified to examine the Monster. Unfortunately before Ludwig could do so, the Monster broke loose and escaped back to Ygor. When the police had searched every inch of Visaria and found no sign of either Ygor or the Monster, Erik began to suspect that Ludwig might be harboring them.
The unexplained disappearance of Dr. Kettering, combined with Ludwig's uncharacteristically hostile attitude towards Erik when he and two police officers arrived to search the Frankenstein estate, seemed to confirm the prosecutor's suspicious. However, their search turned up nothing and Erik was told, quite bluntly, that he was unwelcome at the Frankenstein house after that.