Caius is the bad-tempered French doctor who wants to marry Anne Page. He's also Mistress Quickly's verbally abusive boss and could definitely use a session or two of anger management class. Like, notice how he's always running around picking fights, making outrageous threats, and screaming "by gar!" (aka "by god!").
We know what you're thinking: "By gar! What's up with the thick accent?" Good question, Shmoopers; we're glad you asked. Like Welshman Hugh Evans, Caius's status as a foreigner makes him an outsider and the butt of a lot of jokes in this play, like when he uses the word "turd" when he means to say "third" (3.3.235). In fact, The Host of the Garter Inn spends an entire scene ragging on Caius's French accent and insulting him by using a bunch of English slang that the guy doesn't understand.
You want an example? Of course you do. At one point, the Host calls him "Monsieur Mockwater." As we know, "mockwater" basically means impotent, sterile, or cowardly, so, when the Host calls the doctor Monsieur Mockwater, he's implying that Caius is a wimp because he's French. Of course, Caius takes all of this as a compliment because the Host has told him that "mockwater" is just English slang for "valour." Check it out:
'Mockwater,' in our English tongue, is 'valor,'
By gar, then I have as much mockvater
as de Englishman. (2.3.60-63)
Why is Shakespeare so hell bent on making the Frenchman look foolish? Well, Shakespeare is all about trying to give us a snapshot of small town English life. In the process of trying to define what it means to be English, Shakespeare often uses foreigners like Caius to show us what Englishness doesn't look like. The end result? A play that ends up looking a smidge xenophobic. (Just like a lot of English people were a smidge xenophobic at the time.) Go to "Themes: Language and Communication" and we'll tell you more about this.