When we first meet Dunstan, he's eighteen and pretty ordinary what with his "nut-brown hair, and nut-brown eyes, and nut-brown freckles." He's also "middling tall, and slow of speech" (1.23). But hey, at least he has "an easy smile" (1.23), and generally seems like a nice guy.
When a tall gentleman in a black top hat asks to rent his room for the Faerie Market, Dunstan agrees… and then he asks for a miracle or wonder in exchange. The gentleman grants Dunstan his Heart's Desire, which turns out to be a fling with a faerie slave girl, whom he's totally smitten with upon noticing how beautiful she is. She offers to sell him a glass snowdrop for a kiss, which just ensnares him further:
He smelled the scent of her then, intoxicating, magical; it filled the front of his head and his chest and his mind. (1.151)
Even though Dunstan supposedly bought the glass flower to give to his beloved Daisy (who is as practical as he is), the faerie girl is still on his mind. He gives Daisy the flower, boldly kisses her on the cheek, and spends the rest of the day in a stupor. The faerie girl invites him to spend the evening with her, which he does. He kisses her "tentatively" (1.190), leading them to hook up for the evening.
And that's all Dunstan sees of his faerie lover. He acts all weird and distracted after the Faerie Market, until his mom plots to get him to marry Daisy Hempstock. Their parents conspire to build them a cottage and gift them some sheep to look after, and this apparently helps with Dunstan's recovery. We're told:
They lived in Dunstan's cottage, while their little farmhouse was erected, and they were certainly happy enough; and the day-to-day business of raising sheep, and herding sheep, and shearing them, and nursing them, slowly took the faraway look from Dunstan's eyes. (1.220)
And there we leave Dunstan, a newlywed with his feet back on the ground… until his hook-up comes back to haunt him.
When Tristran is seventeen, Dunstan "at five-and-thirty was still middling tall and still freckled, although there were more than a few silvering hairs in his nut-brown curls" (2.104). In other words, he's still got his totally middle of the road looks from way back when.
He knows where Tristran comes from, so he's not surprised when his son announces that he's going East. More than that, though, Dunstan is willing to help his son head off on his mission. He gets Tristran through the gap in the wall, which is normally guarded, by alluding to Tristran's origins, saying, "'Well, it's true […] And now it's time for him to go back'" (2.132). Once the men on duty assent, Dunstan tells his son:
"Go on with you, boy. Go, and bring back your star, and may God and all His angels go with you." (2.136)
Pretty sweet sentiments, right? He also gives Tristran the glass snowdrop to take with him. This is the flower that led to Tristran's birth, something which Dunstan had not allowed Daisy to wear at their wedding. He probably senses that it's Tristran's birthright, and so it belongs to him now.
When Tristran makes it back, Dunstan is waiting to greet him, and with "pride in his voice" he tells his son, "'So you made it back without hurt'" (10.98). While this isn't technically true since Tristran burned his hand, he did make it back in one piece—and that Dunstan hones in on this right away, suggests he thought of his son and his wellbeing while he was off adventuring.
Dunstan finally tells Tristran the story of his birth, and then they have one last meal together before saying goodbyes. It's good that Dunstan gets a chance to see his son all grown up before Tristran leaves for Faerie for good, since it's pretty clear he loves the guy.