Though a butler is a servant and is therefore not a likely key character in a story, in The Visit by Friedrich Durrenmatt Boby the butler plays a significant role in the story line.
Claire Zachanassian is the protagonist of this story, and she has come back to the town of Guellen (or Güllen, depending on the translation) for a very special visit--thus the title of the play. This is the town from which she was effectively banished for telling the truth.
Unfortunately, she fell in love with and got pregnant by a powerful man in town, Alfred Ill. He was, and still is, a powerful presence in Guellen, and he corrupted the judicial process by bribing witnesses to characterize her as a woman of low morals. Of course her paternity suit was dropped because of this evidence. Ill then married for money and, though he is not exemplary in any way, is set to become the next mayor of the town.
Claire, on the other hand, left in disgrace and was forced to prostitute herself to make a living and support her child. She was working in a brothel when her life changed; a millionaire discovered her, married her, and left her his fortune.
Now she is back in Guellen with a nefarious scheme in mind. She is an imposing and eccentric woman, sixty-three years old now and sporting an ivory hand and an artificial leg. This red-headed woman is exceptionally wealthy, and she is carried around in a sedan chair. She dresses in black and has a coffin with her at all times. She has a black panther and smokes cigars, as well. In all, she cuts quite a figure when she reappears in Guellen.
With her is Boby, her eighty=year-old butler. We do not know it at first, but eventually we discover that Boby is the former Lord Chief Justice of Guellen and the man who presided at the paternity trial between Claire and her young lover so many years ago. Of course he ruled that Ill was not the father of Claire's child because he believed the witnesses that Ill provided.
Boby is a significant character to this play because it is through him--a much more authoritative voice than that of a scorned and rather eccentric woman--that we learn of Ill's long-ago treachery. Through a series of questions and answers (in which he sounds more like a lawyer--or a judge--than a butler), Boby is able to reveal Ill's courtroom maneuvers to the town.
He takes off his dark glasses and announces who he is before telling this story:
[T]wenty-five years ago, Mrs. Zachanassian offered me the opportunity to enter her service as her butler. I accepted. A peculiar career for a man of learning, perhaps, but the salary was so fantastic--
Clair interrupts him because of course she does not want the focus to be on the money; however, the money has to be the focus, at least for us. Boby's revelation is the embodiment of the irony in Guellen's current mayor's adamant statement that "justice can't be bought." Obviously the law can be corrupted (as evidenced by Ill's long-ago actions) and justice can be bought (based Boby's willingness to be "bought").
Boby then calls witnesses and forces testimony which all prove that Ill was to blame not only for perverting true justice for Clair but also for indirectly killing his child who only lived to be a year old. Once Ill has gotten the punishment Claire believes he deserves, she releases Boby from his service to her.
Just as Ill used the corrupt justice system to suit his needs at the time, so does Claire (through her "purchase" of Boby) use the justice system to suit hers.