We hear the star before we see her. She falls into a glade, and a "high clear, female voice" says "ow" and then "F***" (3.105), presumably because of the pain of falling to earth. Her first contact with a human, as far as we know, is when Tristran shows up to fetch her. Her response? Is to chuck mud at him. So, you know, she's not in a good place. In her defense, though, she did just pretty much fall out of her house.
When Tristran finally figures out that she's the star, he goes ahead and slips the unbreakable chain around her wrist. She goes from sobbing and saying stuff like "'go away and leave me alone'" (4.264) to:
"I just want you to know […] that whoever you are, and whatever you intend with me, I shall give you no aid of any kind, nor shall I assist you, and I shall do whatever is in my power to frustrate your plans and devices." (4.274)
Message received, loud and clear: This girl isn't just mad, she's ready to get even. She also starts calling him all kinds of names, ranging from "clodpoll […] ninny […] numbskull […] lackwit […] coxcomb" (4.266), to "Dunderhead. Bumpkin. Dolt" (4.286), and "Cretinous, verminous oaf" (4.287). We can't help but give her points for creativity. Girl is quite the wordsmith when it comes to crafting an insult.
She isn't just sour about captivity, though—she's also pretty scornful when it comes to love. When Tristran tries to explain that he is bringing her back for the love of his life, the star snorts "with derision" (5.7) and asks, "'And this wise, sweet creature sent you here to torture me?'" (5.7). We can't really blame her for being so cynical, given what she's gone through on her fall.
And while we get that Tristran is well-intentioned, it makes perfect sense that she—as his captive—doesn't see him this way. So we can't blame her for running away when she gets the chance, even though it leaves Tristran in a lurch, and it opens her up to all kinds of trouble.
Even after Tristran saves her life at the inn, she's still full of not-so-nice feelings toward him. She tells him:
"Now that you have saved my life, you are, by the law of my people, responsible for me, and I for you. Where you go, I must also go […] I would rather spend my days chained to a vile wolf or a stinking pig or a marsh-goblin." (8.20-22)
She calls 'em like she sees 'em, that's for sure. And even though Tristran knows now that it's not nice to put chains on people, she's bound to him more than ever before. Ugh.
We're not sure where Yvaine learned to curse, since she's not human and all, but she sure looks the part. Basically, she looks like a blond goth girl who doesn't get much sun: "Her hair was so fair it was almost white, her dress of blue silk which shimmered in the candlelight" (4.259). She tends to glitter under a light source at night, and her eyes are "blue" (5.16) and often filled with tears, especially when her leg is freshly broken.
She doesn't need to eat or drink, which makes it tough to pass for human if anyone's paying close attention. And sometimes she says weird things, like that she's never had a bath before (but she says this to the innkeeper's wife, a.k.a. the witch-queen in disguise, so it's not like she's really letting the cat out of the bag).
Interestingly, particularly given her quick and assertive response to Tristran capturing her, because the star is actually rather inexperienced when it comes to humanity, it doesn't occur to her that the innkeeper's wife might mean her harm. She totally falls for the sweet-older-lady act: "There were good people on this benighted world, the star decided, warmed and contented" (7.24). Yeah, good people who want to cut out your heart.
When the star and Tristran come across a lion and a unicorn fighting, she begs him to break up the fight, pleading, "'Please, do something. The lion will kill him'" (5.42). We later learn that unicorns are the moon's creatures, and so are stars, so they share a natural affinity (or something like that). When the lion leaves after winning the fight, the star "hobbled over to the injured unicorn and lowered herself to the grass, awkwardly" (5.50) in order to stroke its head and talk to it while it recovers. Consider her soft spot officially revealed.
As Yvaine becomes more comfortable in Tristran's presence, she starts singing, too, and it's beautiful music, enjoyable to anyone who hears it. So she doesn't just have some niceness deep down inside her—she also starts making the world a more beautiful place to be in.
She eventually comes around to sort of liking Tristran, too, after he saves her life. She tells him:
"I am lucky to have fallen in Faerie. And I think I was probably lucky to have met you." (8.123)
This is when she starts falling for Tristran, and eventually they fall in love and decide to spend the rest of their lives together. We'd say this shows a real knack for forgiveness on Yvaine's part, considering that when she first met Tristran he bound her to him.
By the end of the book, when Yvaine meets up with the shriveled old witch-queen, she "realized that she felt nothing but pity for the creature who had wanted her dead" (10.202). Super generous, right? It's as though Yvaine's ability to have human feelings has been switched on since she's spent so much time on earth. And it's a good thing, too, since when Tristran dies, she becomes ruler of Stormhold.
Despite being a star, Yvaine proves to be a worthy leader on earth. The combination of her human feelings and the instinctive assertiveness and willingness to stand strong that she brought down with her from the sky position her to do right by her people. So while she may never return to the moon, it seems she gets a happy ending anyway.