There's really not that much to Mrs. Jennings. She's a nice lady, but she can be on the annoying side (to put it mildly). Her roots are rather common – her husband was a businessman who made his fortune in a non-genteel way, through commerce – but she doesn't let that stop her from attempting to infiltrate the social circles of the rich and important. She's a social butterfly, whose only joy in life is a good gabfest; whether in London, Cleveland, or Barton, Mrs. Jennings simply has to have a lot of friends, family, and guests to talk to.
Mrs. Jennings generally has the best of intentions, even if she unwittingly puts her foot in her mouth all the time. She loves gossip, and she loves to tease her friends with the things she knows. For example, her favorite topics are Elinor and Marianne's relationships with Edward and Willoughby, respectively; she never seems to know how far her boundaries extend. Propriety, convention, and tact don't really disturb her much, and she simply doesn't know where to stop once she starts. Her biggest flaw is her lack of perception – she's a little slow on the uptake with regards to other people's discomfort or annoyance, and she's apt to push the subjects of her jokes a little too far (especially with regards to Marianne).
However, once Mrs. Jennings realizes that something is wrong, she's an endlessly supportive friend. When Willoughby's engagement emerges, for example, she keeps her promise to Elinor and doesn't bring it up in front of Marianne, and her concern for her young charge is really quite touching. In the end, she's simply a wholeheartedly nice woman, with an unfortunate passion for gossip.