What is the significance of the second traveler?
It is not completely clear who the second traveler is. Even though he is older, they bear a strong resemblance to each other:
As nearly as could be discerned, the second traveler was about fifty years old, apparently in the same rank of life as Goodman Brown, and bearing a considerable resemblance to him, though perhaps more in expression than features. Still they might have been taken for father and son. And yet, though the elder person was as simply clad as the younger, and as simple in manner too, he had an indescribable air of one who knew the world, and who would not have felt abashed at the governor's dinner table or in King William's court, were it possible that his affairs should call him thither.
The fact that Young Goodman Brown and this man could be taken for "father and son" suggests a close relationship. When YGB questions him, the traveler says that he knew his grandfather and his father too:
I helped your grandfather, the constable, when he lashed the Quaker woman so smartly through the streets of Salem; and it was I that brought your father a pitch-pine knot, kindled at my own hearth, to set fire to an Indian village, in King Philip's War.
The traveller seems to have access to secret knowledge about his family that surprises YGB. Could his assumptions about the uprightness of his family, and his friends and neighbors, be wrong? Could his grandfather and father be guilty of such sin?
One could argue that the second traveler is the devil, sent to claim YGB's soul; or that he is an outward manifestation of YGB's own doubts, a symbolic representation of temptation, or an alter-ego, the sinful side of YGB's nature. Whetever his true nature, his significance in the story is to persuade YGB to go into the forest, and to believe the worst about his neighbors and even his wife.
It is said that this second traveler, which is mentioned only briefly in the story, has very similar physical characteristics as YGM, but it is a man of around 50 years of age, and with a strange countenance that leads us to believe that he may be YGM himself, but the reflection of his darker, more mysterious self. In other words, this second traveler is a form of alter ego that represents the inner and darker thoughts of YGM and the ethereal self who is much malicious and has the capacity to break away from a life of purity.