The client is a young woman who is dressed in black and terribly frightened.
Helen Stoner is a young lady “in a considerable state of excitement” when she comes to hire Sherlock Holmes. Of course, most clients are probably not incredibly calm. They are hiring a detective, after all. It means they are in trouble of some kind. However, this one is “dressed in black and heavily veiled.” She is also shivering. Holmes asks her if she is cold. She says it is terror.
[She] was indeed in a pitiable state of agitation, her face all drawn and grey, with restless frightened eyes, like those of some hunted animal. Her features and figure were those of a woman of thirty, but her hair was shot with premature grey, and her expression was weary and haggard.
Holmes tells her not to worry. He deduces that she came by train, but partially by dog cart because he points out that the “left arm of your jacket is spattered with mud in no less than seven places.” She tells him that the horror of her situation is that her fears are vague, and finally introduces herself as Helen Stoner.
Helen Stoner’s problem is that her stepfather killed her sister for the inheritance, and wants to kill her too. Her stepfather is a dangerous man.
Violence of temper approaching to mania has been hereditary in the men of the family, and in my stepfather's case it had, I believe, been intensified by his long residence in the tropics.
Holmes listens to her story, and determines that he needs to go to her home with her in order to solve the case, which he does. The murder weapon? It is a snake.
In this case we see another example of how Holmes deduces a lot of important information about the case only from a person's physical characteristics. People tell us a lot about their circumstances and where they have been by their mannerisms, and the condition of their clothes. It seems extraordinary, but it is just deduction.