The 16-year-old second daughter of Mr. and Mrs. Henry Dashwood, she mostly embodies the "sensibility" of the title, as opposed to her elder sister Elinor's "sense".

When Marianne is helped by the dashing Willoughby, she falls deeply and sincerely in love with him, abhorring all of society's conventions, and ignoring her sister's rational warnings that her impulsive behaviour leaves her open to gossip and innuendo.

Marianne treats her acquaintances in general with inattention and sometimes, contempt, recoiling from vulgarity, even when it is accompanied by good nature (such as with Mrs. Jennings), treating her selfish half-brother and his snobbish wife with disgust, ignoring the grave Colonel Brandon because of his age and a former love, and making no attempt at civility to insipid Lady Middleton.

Marianne's form is "not so correct as her sister's", but "more striking", and her features are all good, her face is "lovely": her skin is very brown, but from its transparency, "her complexion was uncommonly brilliant", and in her eyes there is "a life, a spirit, an eagerness which could hardly be seen without delight".

Later in the novel, still grieving over having lost Willoughby, she ignores her health, falls dangerously ill with a putrid fever, and nearly dies as a result.