Answer: Mary Morstan is a model client because she is calm, intelligent, well-organised and brings Holmes all the facts and evidence in a clear, orderly way.
Mary Morstan, a young governess in the house of Mrs. Cecil Forrester, comes to Holmes about her missing father and the mysterious pearls. She is self-controlled and dignified, and she narrates her case clearly and precisely. She has carefully preserved the pearls, the letter inviting her to a meeting and a strange paper found in her father’s desk bearing the ‘sign of four’. Impressed, Holmes remarks that she has a decided genius for organisation and that she is a model client. She is also simple, refined, sensible and courageous, which wins Dr. Watson’s heart.