This guy is the innkeeper and bartender of Wall. He has a striking appearance compared to the rest of the plain-looking inhabitants of the town: "Mr. Bromios was tall, and his skin was olive; his black hair was curled tightly on his head; his eyes were green" (1.31). We can see where he might seem a little exotic.
He's not a native of the town, but he's been accepted by the locals: "It was said he had come to the village quite some time ago, a visitor. But he had stayed in the village; and his wine was good, so the locals agreed" (1.31). Years later, when Tristran is leaving for Faerie, Mr. Bromios is one of the guards at the wall. Tristran notices that "his smile was white, and he smelled of grapes and of grape juice, of barley and of hops" (2.120). He helps convince Harold Crutchbeck, the other guard, to let Tristran through without a fuss. We think Mr. Bromios knows what's up.
Funny thing about Mr. Bromios: his hair is described as "black" (2.120) both the year when Tristran is conceived, and the year when Tristran, at seventeen, leaves for Faerie. Maybe he's slow to start graying, or maybe there's more to this man than meets the eye. Anything's possible in Stardust it seems.