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This guy only makes a brief appearance at the end of Book 2, but it's an important one. He literally steers the boat that takes the Palmer and Guyon to the Bower of Bliss and so he figuratively represents action, or the will.
In other words, the Palmer does a lot of the thinking and guiding, but the ferryman does a lot of the doing. He represents the importance of not just knowing the right thing to do, but actually going through with it.