Though probably intended as a caricature of the oversensitive heroine in the late-eighteenth-century novel, Marianne is a character in her own right: "She was sensible and clever, but eager in everything; her sorrows, her joys, could have no moderation."
Marianne is amazed that Elinor could love the colorless Edward. "He is not the kind of young man — there is something wanting," she tells her mother. She looks on Colonel Brandon as an old man, past romance, although he is only thirty-five, and falls headlong in love with the shallow Willoughby: "His person and air were equal to what her fancy had ever drawn for the hero of a favorite story." Always too impulsive, Marianne goes with Willoughby to look over Mrs. Smith's house, accepts his offer of a horse, and pokes fun at Colonel Brandon to please him. Intolerant of the feelings of others, Marianne is displeased by Sir John's jests and finds Mrs. Jennings vulgar and gossipy. She treats the old lady impolitely during their trip to London but is eager to avail herself of Mrs. Jennings' hospitality. She is outspoken and honest, and cannot tell even a polite lie: "It was impossible for her to say what she did not feel, however trivial the occasion."
When Willoughby deserts her, Marianne loses all self-control and eventually becomes ill. When she recovers, she realizes that she has brought her troubles on herself, and she admits to Elinor that Willoughby never actually proposed marriage to her. She realizes her faults and how often she has hurt others: "Everybody seemed injured by me. The kindness, the unceasing kindness of Mrs. Jennings, I had repaid with ungrateful contempt. To the Middletons, the Palmers, the Steeles, to every common acquaintance even, I had been insolent and unjust."
At last, learning sense, she appreciates Colonel Brandon at his true worth. Married to him, she achieves happiness because she "could never love by halves; and her whole heart became in time, as much devoted to her husband, as it had once been to Willoughby."