Paroles is Bertram's BFF / wingman / gentleman servant… until a group of guys trick him into revealing that he's a coward and a liar, that is. (We'll explain this in a minute.) Come to think of it, Paroles is like the Stifler (American Pie) of Shakespearean drama. He's raunchy, arrogant, sexist, and all around revolting. He's also the kind of guy Holden Caulfield would refer to as a "phony bastard."
For some, he can also be sort of funny to watch... in a holy-moly-that-guy-is-so-obnoxious kind of way. Remember when Paroles tries to convince Helen to lose her virginity? It's pretty appalling, but it always gets a good laugh from audiences. (Check out a clip of Paroles' big speech virginity speech here.)
In fact, Paroles was one of Shakespeare's most popular stage characters in eighteenth-century productions of the play. According to Shakespeare scholar Alexander Leggatt, during the 1700s, a bunch of productions cut out entire scenes and characters in order to make the play all about Paroles. Earlier than that, King Charles I (1600-1649) obviously thought Paroles was the most important character in All's Well. In his personal copy of the play, he wrote "Monsieur Paroles" right next to the title. Crazy, right? (Source.)
The other thing to know about Paroles is that he acts like a typical braggart soldier, a soldier with a big mouth and a penchant for talking about how awesome he is on the battlefield and with the ladies. (By the way, the braggart soldier character dates back to ancient Greek and Roman literature, like Plautus's Miles Gloriosus, and shows up a lot in Renaissance drama. Think Falstaff and Pistol from Shakespeare's Henry IV Part 1 and Henry IV Part 2.)
The thing about Paroles (along with all other braggart soldiers) is that, even though he talks a good game about being a war hero and a ladies' man, he's neither. First of all, he likes to talk dirty to girls (1.1.128-138), but we never see him hooking up with anyone. Plus, every woman he comes into contact with ends up hating him. Helen calls him a "fool" and a "coward" (1.1.106), Mariana calls him a "filthy officer" and a "knave" (3.5.18, 17), and the widow screams out "Marry, hang you" when she sees him on the street (3.5.107).
Paroles is also a chicken when it comes to warfare. This becomes clear when he loses his drum on the battlefield (kind of a big deal since drums were used to convey orders during battle). When his friends dare him to go back and get it (3.6.41-43), he stands around trying to come up with a story to convince his friends that he was injured. This blows up in his face when his pals show up disguised as enemy soldiers. After Paroles is captured, it takes him about two seconds to sell out his friends in exchange for his freedom. (This is a lot like what happens to Falstaff, whose friends stage a fake robbery to humiliate him in Henry IV Part 1. It also reminds us of what happens to Malvolio in Twelfth Night.)
The point is this: Paroles is all talk and no action, which is probably why Shakespeare named him after a French term (parole) that means "words" or "speech."
Paroles is the countess's worst nightmare, especially given that he has so much influence over her son, Bertram. Think about it. Paroles encourages Bertram to ditch his wife (2.5.21, 23) and also acts as a go-between when Bertram tries to have an extra-marital affair with Diana. Plus, he's always giving Bertram silly advice and encourages him to act fake so he can fit in at the French court: "there do muster true gait," he says. "Eat, speak, / and move under the influence of the most received star" (2.1.61-62).
Okay, it's completely obvious that Paroles is bad news. But is he responsible for the way Bertram acts? When the Countess finds out that her son has run away to Italy, she blames Paroles for "corrupt[ing]" her son, calling him a "very tainted fellow, and full of wickedness" (3.2.94). Still, is Paroles just a convenient scapegoat for Bertram's bad behavior? After all, Paroles never forces Bertram to do anything and Bertram is old enough to make his own decisions... don't you think?