Mephisto

For party members, a devil. He was (according to his own words) initially alerted to their presence when Rodrick took a hair ornament from Mutti Courage, and wore it in his own hair. Shortly after, he and his companions began to encounter a small black terrier in the woods. 
At one point, with only Mechtild present, the dog was rooting around in Rodrick's pack, and seemed particularly interested in one of his magical books. Rodrick later discovered some writing in the book that he had not seen before. It said: "call me, and I will come". Rodrick, suspecting that the dog was Reinhard in disguise, asked what his name was, and the word "Mephisto" appeared in the book. 
Rodrick and Mechtild went alone into the woods outside of Drachenstelle, and Rodrick intoned the name. A gentleman in red then appeared. When asked whether he was Reinhard, he denied this, though he seemed to know the latter well. He said that Reinhard was sent to watch the river, and wanted to know why he left. He and his interlocutors quickly established that the cause was a woman - specifically, Winifred.  
Mephisto then got down to business, and asked Rodrick what he wanted, since he summoned him. Rodrick wanted to know what his hair ornament did, and Mephisto told him that in addition to making him known to Mephisto, it also provides the wearer preternatural toughness, as if the wearer were an orc. Rodrick then got cold feet, and said he would have no further dealings with Mephisto, and would not invite him to the castle. Mephisto said it was already too late, as he summoned him on the territory of the estate. Mechtild then piped in, and asked what the cost would be for gaining the ability to distinguish truth from lies. Mephisto, after rejecting the suggestion that he was interestd in body parts as payment, said that he could grant this power in exchange for Mechtild bringing Winifred to him for a talk in the woods. Mechtild agreed, and the bargain was sealed with her blood. Then, she and Rodrick returned to the castle.