We get a pretty different sense of Bridget's character at the beginning of the novel and at the end. At the start of Tom Jones, Bridget Allworthy, the squire's sister, comes off as an opinionated older woman (she's about forty) who is really nasty about younger women who have lots of sex.
When she first sees baby Tom, she thinks he's adorable. But as for Tom's absent mother, Bridget calls her an "impudent slut, a wanton hussy, an audacious harlot, a wicked jade, a vile strumpet" (1.4.12). This is pretty tough talk about a woman Bridget doesn't even (apparently) know. Bridget seems pretty rigid in matters of sexual morality, and she also appears dismissive and judgmental towards younger women who are less prudish than she is.
The narrator describes Bridget as looking like this woman in artist William Hogarth's print "Morning." Notice how the well-dressed but sour-looking woman in the print just stands and watches younger women frolicking with men? Bridget is like that: she never participates in any kind of relationship with a guy, which makes her doubly paranoid about and fascinated by other people's sexuality.
However, Bridget doesn't stay an unhappily unmarried woman. Soon, she becomes an unhappily married woman. One of her brother's friends, the penniless but wily Captain Blifil, arrives on the scene to woo her. He knows that Bridget is Squire Allworthy's heir, and that her children will inherit the estate when the squire dies. Captain Blifil likes the looks of Bridget's future money (even if he doesn't think much of Bridget herself) so he elopes with her. The two of them have a child, Henry Blifil. (We like to call him either Master Blifil (when he's a kid) or Mr. Blifil (when he's an adult) because that's what the book calls him.)
It doesn't take long for Bridget's feelings for the captain to wear off. Practically as soon as that baby is in his baby carriage, Mr. and Mrs. Blifil start earnestly and truly despising each other. What initially brought them together—their shared learning on religious matters—quickly drives them apart again. Bridget thinks the captain is a bossy idiot, while the captain dismisses all of Bridget's strong opinions on religion and morality as the words of a foolish woman.
The only thing that repairs Bridget's opinion of Captain Blifil is that he dies suddenly; in his absence, she starts to like him better. (Wow, this book is cynical, sometimes.)
As her son grows up, Bridget seems to love him less and less. Considering how much she hated his father by the end, perhaps it's unsurprising that little Master Blifil is so loathsome to Bridget. But she does take a shine to young Tom Jones, so much so that the local people start to gossip that maybe something is going on between the widow and Squire Allworthy's handsome young ward. (There isn't. But it is important to notice Bridget's pro-Tom vibes!)
Bridget dies suddenly in Book 5, while she's on vacation away from home. Given her sudden disappearance from the story, it makes sense that we basically stop thinking of her until the end of the book. It feels like we have a pretty complete portrait: she is a vain and judgmental woman who doesn't seem cut out for family life, what with the way she despises her son and all.
We were honestly surprised, in Book 18, Chapter 7, when Jenny Jones/Mrs. Waters reveals that Bridget Allworthy is actually Tom Jones's mother. We guessed that Jenny Jones probably wasn't the mother, since that seemed too easy an explanation. But Bridget? This information puts a whole new spin on her description of Tom's mother as an "impudent slut" (1.4.12). Far from being hyper-judgmental of other women's sexuality, she was just trying to distract everyone else from her own out-of-wedlock sexy times!
Bridget is one of the only characters we can think of in this novel who turns out to be other than what she seems, even to the reader. There are lots of hypocrites and liars in Tom Jones, from Black George all the way up to Lady Bellaston. But even if other people in the books are fooled, we the readers know that they are up to no good.
By contrast, to us, Bridget seems opinionated and judgmental in the earlier books of the novel. We never dreamed that she had been sneaking off for some hanky-panky of her own, with a young man named Summers, who lived at Squire Allworthy's house for a time. Maybe it's because the narrator appears so trustworthy and all-knowing in his descriptions of Mr. Blifil and the novel's other hypocritical characters that we were completely fooled with this one exception, when he doesn't tell us what's really going on beneath Bridget's surfaces.
The sudden reveal of Bridget's true relationship to Tom explains why she was so close to him before she died. It also makes us wonder what Bridget was really like on the inside, and what made her do the things she did.
Squire Allworthy mentions that he offered to arrange a marriage between Bridget and Summers. If she liked Summers enough to have his baby, why didn't she agree to her brother's kindly suggestion? And Mrs. Waters claims that Bridget always meant to tell her brother everything. It just didn't seem urgent to her, because Squire Allworthy loved Tom even without knowing that Tom is his nephew by blood. Even if that explains why Bridget didn't tell Squire Allworthy (which we sort of feel it doesn't), why didn't Bridget ever talk to Tom about his true parentage? We'll never know the answers to these questions, which is a shame.