Final answer:
The character described as a 'little sallow, rat-faced, dark-eyed fellow' refers to Dr. Grimesby Roylott, the antagonist in 'The Adventure of the Speckled Band' by Sir Arthur Conan Doyle.
Step-by-step explanation:
This description is not directly associated with Sherlock Holmes, Dr. John Watson, or Professor Moriarty, who are all characters in the same series. Instead, Dr. Grimesby Roylott is portrayed as the antagonist in 'The Adventure of the Speckled Band', which is one of the stories within the collection.
Dr. Roylott is the stepfather of Helen Stoner, the client in 'The Adventure of the Speckled Band', and he is depicted as a villainous character. The description matches his unpleasant physical appearance and menacing personality. Although Doyle's characters are fictional, he often gave them distinctive features to highlight their roles in his stories. Dr. Roylott's description as a "little sallow, rat-faced, dark-eyed fellow" is indicative of his malevolent nature and serves as a tool to invoke a specific image and foreboding presence in the reader's mind.