* Sherlock Holmes: The brilliant detective who is tasked with stopping Milverton.
* Dr. John H. Watson: Holmes' loyal friend and chronicler of his adventures.
* Charles Augustus Milverton: The villain of the story, a blackmailing fiend who uses private information to extort people.
* Mrs. Hudson: The landlady of 221B Baker Street where Holmes and Watson reside.
* The Woman: A mysterious woman who appears to be connected to Milverton and is later revealed to be his victim.
* The Butler: Milverton's servant, who acts as a gatekeeper and protector of Milverton's secrets.
* The Police Inspector: A representative of the law who is initially skeptical of Holmes' methods.
* Lord and Lady Esher: A wealthy couple who are being blackmailed by Milverton.
There are also several minor characters who play smaller roles, such as the servants and other individuals who are mentioned in the story.