Johann is a gamekeeper in the service of Black Michael, Duke of Strelsau. He first helps Rassendyll by telling him of the arrangements the duke has made to prevent the king's body from being recovered if he should be killed. The murderer will attach weights to the king's corpse before sliding it down a pipe into the moat.
Rassendyll is able to incorporate this information into his rescue plans and also learns from Johann when the king is growing ill, meaning that the rescue must take place as quickly as possible. Johann is initially decoyed into the service of Rassendyll and his friends (and thus, indirectly, the king), but they promise him various rewards and inducements to assist them, and he comes over to their side quite readily. He particularly dislikes Rupert of Hentzau, who has had him flogged, and Rassendyll remarks that this beating "and the duke’s approval of it did more to bind the keeper to my side than all my promises."
Johann continues to supply Rassendyll with information throughout the story, and it is part of the plan that he should assist in the rescue by opening the door of the castle for the rescuers. However, at the last minute, he is wounded and prevented from doing so. His information nonetheless proves to be correct and vital for the success of the rescue.