When Dunstan first sees this woman at the Faerie Market, she seems very striking, and very obviously not human:
[…] she smiled widely at him with white teeth in a dusky face. She was one of the folk from Beyond the Wall, he could tell at once from her eyes, and her ears, which were visible beneath her curly black hair. Her eyes were a deep violet, while her ears were the ears of a cat, perhaps, gently curved, and dusted with a fine, dark fur. She was quite beautiful. (1.122)
He also notices the "thin silver chain that ran from the young lady's wrist, down to her ankle and into the painted caravan behind her" (1.135). So she's not there of her own free will, which is a bummer. Dunstan seems sympathetic, and asks what happened to her.
She tells Dunstan:
"I am the personal slave of the witch-woman who owns the stall. She caught me many years ago—as I played by the waterfalls in my father's lands, high in the mountains-luring me on and on in the form of a pretty frog always but a moment out of my reach, until I had left my father's lands, unwittingly, whereupon she resumed her true shape and popped me into a sack." (1.137)
That's pretty crafty, and also a major bummer for the slave girl.
She'll get to be free eventually, though, "on the day when the moon loses her daughter, if that occurs in a week when two Mondays come together" (1.139). Does that sound nutty to you? Never fear, it's actually totally possible, so she'll get her freedom eventually.
Whether out of loneliness, or to try to set things in motion to win her freedom, she invites Dunstan to spend the night with her. While telling him about her life, she begins to cry. Dunstan kisses her, and she takes it from there: "She wriggled and writhed beneath him, gasping and kicking, and guiding him with her hand" (1.94). Obviously she knows how to get what she wants, and she wants Dunstan, at least for the night.
That's all we see of her until much later, when her son with Dunstan, a.k.a. our main man Tristran, sets out into Faerie.
Otherwise, she spends most of her time in the form of a beautifully colored tropical bird, "as large as a pheasant, but with feathers of all colors, garish reds and yellows and vivid blues" (8.127). She only returns to human shape when her mistress—Madame Semele—needs her to do something at a fair.
When their caravan reaches the meadow outside of the wall to set up for the Faerie Market, Madame Semele returns her to her shape, and "she darted a glance at Tristran, and there was something about those violet eyes that Tristran found utterly familiar" (9.51). By now, she's seen Tristran's glass snowdrop, so she's probably put together the puzzle pieces and figured out that he's her son.
As the chain binding her grows translucent, Madame Semele accuses her of doing something. She responds: "'I have done nothing; nothing that I did not do eighteen years ago'" (10.111). This leads us to think that she wasn't just hooking up with Dunstan for fun (though that was probably part of it). Instead, perhaps she knew that by having a kid, she could help set events into motion that would free her. After all, she'd been stuck for decades already, so why not shake things up a bit and try something new?
When Lady Una regains her freedom and her name, she finds Tristran and tells him to ask for what Yvaine carries: the topaz on a chain, the Power of Stormhold. Next up, she plans to purchase transportation to take them all back to Stormhold in style. She says:
"Now, I plan to barter […[ for a palanquin to take us back to the Stormhold, for we must arrive in style […] We must have bearers, and outriders, and perhaps an elephant—they are so imposing, nothing says 'Get out of the way' quite like an elephant in the front […]." (10.188)
Tristran isn't keen on this idea, but his mom is dead-set on having a palanquin, so she ends up going ahead to Stormhold alone (but in style).
Since Tristran is off adventuring, Lady Una rules as regent. She apparently does a good job, since "the dominions on and about Mount Huon prospered and flourished under her command" (Epilogue.3). After being enslaved for over sixty years (10.188), it's great that she gets to come home and enjoy some of the privileges of her royal blood.