On a general level, the man in decent attire is Young Goodman Brown's companion on this unknown journey. This strange man also becomes a link to Brown's past, as the man explains that Brown's father and grandfather had made similar journeys.
It does become clear through imagery and symbolizm that the man in decent attire is, in fact, the devil. It is through this fact that Goodman Brown learns that all the people he regarded in such high esteem have their own links to the devil (Goody Cloyse, the minister, and the deacon).
The devil could then represent Goodman Brown's ability to give in to temptation and evil; therefore, having his own links with the devil.