The Sergeant of the Law is the medieval version of a lawyer, and a pretty good one if Chaucer is to be believed. How do we know he's good at what he does? Well, he does all the things lawyers are supposed to do: he speaks well, writes an air-tight contract, and knows his case law by heart – about 400 years of it (from the time of William the Conqueror) to be precise! So successful is he that he's often appointed by the king as a judge in the court of assizes, a sort of Supreme Court for criminal law. This professional success seems to have led to great financial success as well, for we learn that nowhere was there so great a "purchasour," or land-buyer, as the Sergeant of the Law, and that all this land is "fee simple," to him, meaning that he owns it free and clear, without having to rely on loans.
By using his professional and financial success to purchase land for himself, the Sergeant of the Law becomes a real social climber – owning land was a surefire way of catapulting your descendants into the nobility. And since the nobility already seem to like this guy (appointing him to plum positions in the court system) his chances of becoming one of them soon seem even more assured. The Sergeant of the Law, then, provides an interesting contrast to the Merchant: with him, we have someone who is using his profession to launch himself into the upper class rather than forming an entirely new, "bourgeois" class like the Merchant and his peers do.
As in the Clerk's portrait, the depiction of the Sergeant of the Law is fairly neutral. Yes, there's a little bit of criticism – we learn that the Sergeant seems busier than he really is, suggesting that he's trying very hard to look like he's earning his paycheck when, in fact, he might be kind of lazy. But the generally favorable impression we get of the Sergeant of the Law from what we learn of his competence balances out this gentle satire.