The central character of Behind a Mask is Jean Muir. Jean is not merely the ruthless, vengeful perpetrator of a plot to get ahead. She is also a remarkably observant woman, with keen powers of analysis and judgment that can turn disadvantages into steppingstones. Jean draws upon what talents she has, and these are superlative. Her consummate acting skills become obvious at the end of the first chapter, when after convincing a group that she is an innocent nineteen-year-old, she adjourns to her room and adopts an expression of fierce disdain. "Come, the curtain is down," she declares, "so I may be myself for a few hours, if actresses ever are themselves."

Jean undergoes an amazing metamorphosis. She takes from her head "the long abundant braids," wipes "the pink" from her face, takes out "several pearly teeth" and undresses to emerge "a haggard, worn, and moody woman of thirty at least." Jean is a remarkable fictional creation partly because she is nothing like the sentimental, domestic Victorian she impersonates, and partly because she has an unknown past. During her private unmasking, the reader becomes aware of a "newly healed wound" that seems to symbolize a psychological one. Jean creeps to bed "like one worn out with weariness and mental pain."

Jean is remarkable, too, for her ability to play to each member of her audience. She times her words and gestures perfectly, staging songs, swoons, tears, silences, or ecstasies which draw out the best or the worst in each Coventry, as well as entrap. "The arrival of Miss Muir seemed to produce a change in everyone, though no one could have explained how or why." Among those changed are shy, sixteen-year-old Bella, a "rosy little creature," who is enchanted by the tutor who sings so well. Bella's mother, Mrs. Coventry, sickly and peevish, is "quite won by the modest, domestic graces of the new governess."

Bella's cousin Lucia Beaufort, who oversees the servants with great care to assure they pay due respect to rank, is subdued by Jean's art but still hostile. Lucia is upset by Jean's power over Gerald, the man she loves and heir to the family fortune. Lucia is Jean's foil, not brilliant or accomplished, merely jealous and passive. When a letter is discovered in which Jean writes, "Bah! how I hate sentiment," Lucia refuses to believe a woman could write such a thing.

Edward, the younger son, aged twenty-one, is the first man drawn to Jean. "Poor little woman! She has had a hard life." Since she sees that he will not be attracted by a "silly coquette," Jean blends a sisterly friendliness with quiet dignity. In one episode she impresses Edward by charming his horse, a distrustful beast, by calmly ignoring it. Jean energizes Edward to pursue the commission he desires in government service. Edward grows love-struck, and at one point physically attacks his brother Gerald as a rival for Jean's affections, explaining afterward in remorse: "She would make a man of me. She puts strength and courage into me as no one else can."

Gerald is indolent, cool, snobbish, a tougher target. He suspects Jean's swoon upon arrival may well be staged. Eventually Jean uses the same stratagem that worked on the horse, charming Gerald by her neglect and enlivening him by her dislike of his indolence. He falls before "the indescribable spell of womanhood" after she casts him in the tantalizing role of her lover during an evening of amateur home theatricals. Having managed to her advantage the simple natures of Bella and Mrs. Coventry, the passion of Edward, the romantic side of Gerald, Jean moves on to the title-holding uncle.

Sir John Coventry is a hale, handsome, distinguished man of fifty-five, who lives in lonely splendor on his estate. His slighting manner toward Jean when she is believed a "mere" governess, at which Jean bites her lips "with an angry feeling at her heart," turns into good will at the mention of her supposed rank. Jean flatters the man, evokes his curiosity with "a charming air of maidenly timidity and artlessness." Still his basic decency slows the process. His reticence, and the threat of imminent exposure, reveal Jean's very human side. "Has all my skill deserted me when I need it most?" Yet her nerves of steel prevail. Jean finds a way to "make him understand, yet not overstep the bounds of maidenly honesty."

Jean's deception of Sir John peels away some of the layers of ambiguity in her character, the effect of her acting. Her true self feels "a touch of genuine remorse" in the presence of his offer. Despite her unscrupulous methods, Jean possesses a real sense of fair play. It shows in her dealings with those family members shattered by the knowledge of her duplicity. Jean rewards democratic Edward and Bella with true "grateful warmth" and promises to repay their kindness. "To you I will acknowledge that I am not worthy to be this good man's wife, and to you I will solemnly promise to devote my life to his happiness." She reproves snobbish Gerald and Lucia, whom she leaves to themselves. Jean does not scheme to rob or disgrace the family, but to expose false attitudes and show the good offices an intelligent, free spirited woman can contribute.