Miss Mary Sutherland is said to have a "preposterous hat and...vacuous face." She is a lady of some small fortune of "a hundred a year," left her by her uncle, who makes a living by taking typewriting jobs at home. She is an as yet unmarried young woman who is gentle mannered, good-hearted, sincere and innocently simple in her expectations. She is not greedy because she lets her mother and new husband, Mary's stepfather, have the use of her annual income while she lives at home until she marries, and she would give up all to know what happened to Hosmer Angel.
When Holmes first meets her, he notices that she has "short sight" and wonders if it is strain for typewriting, never having been previously informed of either fact, of course. She reveals herself to be industrious and hardworking because she lives off her earnings from typewriting while her mother has the use of her income. She is independent of thought and action because she went to the Gasfitter's Ball even though told not to go by Mr. Windibank her stepfather, and she went to Sherlock Holmes for help when told not to go by the same Mr. Windibank.
What she tells Holmes of her courtship with Hosmer Angel shows she is innocent in her expectations; trusting of others' sincerity; easy to influence since she was engaged from the the first walk she took with Mr. Angel; confident in the upright goodness of those she associates with; naively trusting and loyally devoted. Even though there are such odd circumstances surrounding her situation with Mr. Hosmer Angel, like not having his address, sending all her letters to post office, receiving typed letters with equally typed signatures, she implicitly believes and trust Mr. Angel, she keeps her pledge of love and trust.