The miller, Symkyn, is the type of guy who hangs around bars looking for a fight, which is probably what the narrator means when he calls him a "market-betere" (82). He's always ready should the opportunity for a fight occur, carrying with him multiple daggers and a long sword. Because of his lust for violence and the fact that he's supposedly rather good at it, all of the other townspeople are afraid of him. Nobody wants to upset Symkyn. That's probably why he's been able to make a tidy profit at his mill by cheating his customers, presumably by "fixing" his scales or padding out sacks of flour and meal with a less-expensive substance, like bran.
The narrator tells us that Symkyn is as proud "as any pecok" (72), which explains why he's so determined to get the better of John and Aleyn when they try to prevent him from cheating them. He takes their presence as a challenge, assuming (rightly) that they think they're smarter than him because they are educated scholars, while he is only a poor miller. Symkyn probably sees the conflict between them as bigger than themselves. He feels like he's representing everyone of his class and that he's taking a stand against the uppity scholar-types who think peasants are stupid and beastly. He makes the rivalry between the two groups explicit when he promises to trick Aleyn and John in spite of "al the sleighte in hir philosophye" (195). Symkyn draws a contrast between book-learning and street-smarts, which he believes he possesses in full, maintaining that "the gretteste clerkes [scholars] been noght wisest men" (200).
Despite Symkyn's determination to prove Aleyn and John wrong about him, he's described as looking like a stereotypical medieval peasant. Symkyn's "camus nose" and round face would be relevant to a scholar of medieval physiognomy, the "science" of learning about someone's character based on their physical features. These physiognomists would point to these physical features as signs of bestiality or excessive physicality, which was part of the medieval stereotype of peasants. Symkyn further confirms an excessively physical nature in his love of violent, physical confrontation.