This guy's a real gem. A retired lawyer, he's Madame Aubain's friend. He takes care of her affairs, but also finds reason to "closet himself with her in Monsieur's study for hours on end" (2.24). Remember—we're talking about the nineteenth century here, so you can add a wink and a nudge to that comment and you'll know what was really going on in that study. Bow chicka bow wow.
Bourais is seen as an intellectual among the town middle class:
He had a great fear of compromising himself, an infinite respect for the law, and a smattering of Latin. (2.24)
This means he's more educated than anyone else, which gives him quite the advantage. And it turns out he uses that advantage to Madame Aubain's detriment.
After Bourais dies, all the dirty laundry comes out:
Rumors of suicide were confirmed, and doubts were raised about his honesty. Madame Aubain studied her accounts and soon discovered the extent of his evil deeds: misappropriation of arrears, secret sales of wood, false receipts, and so on. In addition, he had an illegitimate child, and had had 'relations with a woman in Dozulé.' (4.39)
So it turns out that Mr. Respect-for-the-Law is actually a con man. Par for the course.