When Dunstan Thorn rents out his cottage for the Faerie Market and ends up sleeping in the cow byre, he has an unexpected visitor at night. Someone comes in to take shelter from the rain, and in a flash of lightning, "Dunstan saw something small and hairy in the corner, wearing a large floppy hat" (1.79). This is all Dunstan sees of him, and the small hairy person asks Dunstan's permission to crash there. Permission granted.
But we see him again, though still not in much detail. His path crosses Tristran's when Tristran first walks into Faerie. He shares his food (fried mushrumps, whatever those are) with Tristran, and gives him some pretty valuable advice, too. And then new clothes, and a magical transportation candle… Hey, wait… this dude is actually pretty helpful and hospitable. It's partly because Dunstan gave him crash-space back in the day, but it also helps that Tristran gets him out of a scrape with the serewood.
There might be more to this little fellow than meets the eye, though. For one thing, he doesn't really have a name (though he introduces himself as "Charmed," but that could refer to the writing on the side of his pack). He also makes a reference to "castling" while in the serewood (4.71), which you should totally read up on over in the "Symbols" section.
By the time Tristran has gobbled up this dude's mushrumps and observes his "largely, floppy hat and a large, flappy overcoat" (4.16), Tristran has noticed something. This man—"if man he was, which Tristran found rather unlikely" (4.18)—is probably not human at all.
He has "eyes like beads of jet" (4.41), and it's suggested that he has a "muzzle" (4.45) instead of a chin; he also has a "pawlike foot" (4.71). He refers to Tristran (and presumably all humans) by saying, "'And your sort of people could never learn to burrow'" (4.71), and when he smiles, he shows "small, sharp teeth" (4.103). Survey says: Not human.
But what, then, is he? The author never weighs in definitively, so our guess is that he's an enchanted critter of some sort, that has rodent-like or bear-like characteristics but can also pass for human. He does speak English, after all.
Tristran asks after him at the Faerie Market where he and Yvaine end up together, but while many people have heard of him, no one's seen him recently. So, like many helper figures in fairytales, he remains mysterious and elusive. Color us surprised.