The client in “The Adventure of the Copper Beeches” is Violet Hunter.
Violet Hunter is a governess who visits Sherlock Holmes for advice after receiving an unusually lucrative job offer from Jephro Rucastle. The position requires her to cut her hair, wear a particular electric-blue dress and sit conspicuously by a window at the Copper Beeches estate. Although Holmes warns her to contact him if she becomes endangered, she accepts the post.
At the house, Hunter notices disturbing clues: a locked and neglected wing, a threatening mastiff, strange behaviour from the Rucastle family and a man watching the property. She eventually discovers that Rucastle is holding his daughter Alice prisoner because of an inheritance dispute. Hunter was hired to impersonate Alice and make it appear that Alice had no interest in seeing her fiancé.
Mary Sutherland and Helen Stoner are also Holmes clients in other stories, which makes them plausible distractors. Violet Hunter, however, is the central client and narrator of this particular case. The story first appeared in The Strand Magazine in 1892.