This guy is described as "the tall gentleman in the black silk top hat" who came to Wall "all the way up from London" (1.52). He offers to share his dinner at the (full) inn with Dunstan, and then asks if Dunstan will rent him crash space for the duration of the Faerie Market. Dunstan agrees, and lets the guy sleep in his cottage… for a price.
He offers Dunstan a chunk of cash, but Dunstan holds out for "miracles and wonders" (1.68) instead. The gentleman finally agrees to give Dunstan his "Heart's Desire" (1.70) the next day. He tells Dunstan at the market, "'my gifts last a long time. You and your firstborn child and his or her firstborn child […] It's a gift that will last as long as I live'" (1.99). By now we're figuring out that this gentleman isn't a normal mortal dude, so his lifespan might be ridiculously long.
When Dunstan meets the booth worker selling glass flowers, the gentleman passes by, saying, "'My debt to you is settled, and my rent is paid in full'" (1.126). So he's figured out that Dunstan's Heart's Desire has to do with this beautiful woman, and he's helping their, ahem, union along in some fashion.
He shows up again two Faerie Markets later, and stands near Tristran and Yvaine during a sudden rainstorm. Tristran smiles and nods at him before the couple moves on, and the gentleman says, "'Which is all the thanks I shall ever get from them, I'll wager'" (10.142). Clearly his gift to Dunstan is still in effect (Tristran being his firstborn and all), and so Tristran really ought to be thanking him for how awesome his life turned out, except that this guy's even more of a mystery to Tristran than he is to us. And that's saying something.