We know the merchant is the fashionista of the group because he's wearing a cloak of "motley" (variegated, colorful pattern), a Flemish beaver hat, and has a forked beard, all of which were current fashions at this time period. The merchant's main motivation is "th'encrees of his wynnyng" (General Prologue 277), or making money, and the military protection of trade routes that allows him to do so – these subjects are all he talks about. Although he appears prosperous, we know that the Merchant is actually in debt. He manages to keep his head above water by borrowing more and more. In medieval England, to be in debt was a sign of weak morals. So when Chaucer tells us that the Merchant was a "worthy man withal," we can probably take that a bit ironically.
In the Merchant's Prologue, we learn that he is unhappily married to a shrewish woman who could win a fight against the devil. This state of affairs motivates him to tell a tale about a man who ignores his friend's advice against marriage and comes to regret it.
With the Merchant we have our first example of the "new" up-and-coming middle class. The Merchant's more cosmopolitan sensibilities – knowledge about the latest international fashions, for example – are probably meant to serve as a contrast to the concerns of those like the Knight, who hail from more traditional and entrenched social groups.