In "The Adventure of the Speckled Band," what kind of character is Helen Stoner?

Helen Stoner in "The Adventure of the Speckled Band" is a relatively flat character—to an extent, she could even be called a "stock" character, in that she represents a damsel in distress figure who comes to Holmes in order to present him with a mystery to be solved. She serves an expository function; part of the story, which is mainly narrated by Watson, is narrated by Stoner, as she explains the events which have brought her to Baker Street. The structure of this story echoes that of many of Doyle's others, in which a client comes to Baker Street and sets out their case to Holmes.
There are hints that Stoner is not an entirely reliable narrator in telling her own story. Holmes notices that she has been "cruelly used," spotting bruises on her arm and suggesting that she has been "screening" her stepfather in the telling of her tale. As such, Holmes does not pry further, but suggests that he and Watson should proceed to Stoke Moran in order to ascertain further details himself.

Helen Stoner is a typical young middle-class Victorian woman. She has always led a sheltered, protected, domestic life. She comes to Sherlock Holmes because she wants guidance. The fact that she is so frightened and helpless appeals to the compassionate side of Holmes' nature. He will help her even though she can't afford to pay him. Her stepfather, Dr. Grimesby Roylott, controls most of her resources. Helen Stoner might be termed "a maiden in distress." Holmes often gets involved with young women who are in need of help. His desire to help them provides his motivation which drives the story. Other examples of maidens in distress in Sherlock Holmes stories can be found in: "The Adventure of the Solitary Cyclist," "The Adventure of the Copper Beeches," and "A Case of Identity."
Helen Stoner is not a particularly well-rounded character. She serves mainly to involve the great detective in a perplexing case. She also gives Holmes a great deal of information in the backstory she tells when she first comes to Baker Street. Holmes not only saves Helen's life but solves the "locked room murder mystery" of her sister Julia's death two years earlier.