It is the devil that Goodman Brown meets in the forest. The devil is characterized as looking like an older version of Brown himself "perhaps more in expression than features." He greets Brown familiarly and has, in fact, been expecting him.
The devil is dressed in much the same manner as Brown, and the narrator observes that "he had an indescribable air of one who knew the world," implying that he understood humanity's dark, inner inclinations. He attempts to make Brown feel more comfortable in his presence by claiming that he had been well-acquainted with his father and grandfather as well as eminent men of the colony, including deacons, selectmen, magistrates, and the governor.
As they walk along together, the devil works to convince Brown that he has many converts among the Puritan community, including Brown's catechism teacher. The devil's arguments are effortless and continuous as they walk along, and he exhorts Brown to quicken his pace and to "persevere in the path."
Overall, the devil is characterized as all-knowing, persuasive, and self-assured.