As another answer noted, Bridget is based on Charles' ten-year-older sister, Mary, who acted as a mother to him in a highly dysfunctional family. In real life, Mary stabbed their mother to death in a fit of rage and was placed in an insane asylum. Charles was able to get her out by pledging to support and care for her, which he did for the rest of his life, working, as was common then, six days weeks with only one week of vacation a year. He never married, and like the fictional Bridget and Elia in the essay, Mary and Lamb lived "in a sort of double singleness."
In this essay, Lamb paints a portrait of Bridget, a woman who is based on Mary but does not share her troubled past. He describes Mary as a gentle woman who loves to read—but not the seventeenth century literature that Elia prefers. Mary likes modern works and works with narratives: stories she can get involved in. Bridget can be opinionated, as can Elia. He is usually right about facts, but when it comes moral issues, she is most often able to persuade him to her point of view. Fortunately, he says, Bridget has not been puffed up by a fancy but pointless female education and so remains a simple soul:
she happily missed all that train of female garniture, which passeth by the name of accomplishments.
Bridget is more driven by feeling than Elia, but despite some inevitable clashes and a tendency to over involve herself in other people's problems, the twosome get along well. Bridget is a good companion on outings, such as to the theatre, but is especially fine as travel companion. From this, Elia is able to seque to the trip they took to see relatives in Mackery End, in Hertfordshire.
Lamb was a Romantic, a close friend to people like Coleridge (who he went to school with) and Wordsworth. He quotes Wordsworth in the essay, and like Wordsworth and the other Romantics, places a great value on memory. He cherishes his own fleeting memories and the memories Bridget has of Hertfordshire, which make her girlish again despite being in her fifties.
As a Romantic, too, Lamb shows a simple soul like Bridget (and Elia) in a positive light, writing in a heartwarming away about their ordinary yet intellectually and emotionally rich life together.