This should be a no-brainer. The dude with the staff is a guide because he, literally, guides young Goodman Brown through the forest. Duh.
But things are never no-brainers in Hawthorneland. Most mentors lead us toward wisdom and goodness. This fellow leads young Goodman Brown toward the embrace of sin and a future full of dreariness.
But there is a silver lining. The traveler with the staff gives young Goodman Brown something to push back against. Our hero learns something about how to argue, and a few things about himself, by opposing his guide's opinions. Maybe not the world's best mentor, but not 100% useless, either.