Yes, folks, that's right: this story features two Yseuts, both of them beautiful princesses who are romantically linked to Tristan. Hey, it's the twelfth century; we guess names are scarce. Anyway, thinking he'll never see Yseut the Fair again, Tristan marries Yseut of the White Hands, a princess of Brittany, during his exile from Cornwall. But in a last-minute change of heart, he declines to consummate his marriage with her out of loyalty to the first Yseut. Smooth, buddy. Real smooth.
So, yeah, things aren't that great for Yseut of the White Hands. Like the first Yseut, she is "given" to Tristan without any say in the matter, and then she ends up with a dude who's in love with someone else. She complains to her brother, Kaherdin, about this, saying that a splash of water on her thigh has taken more liberties with her than her husband. You tell 'em, White Hands. At first, Kaherdin is mad at Tristan, but after Tristan shows him Yseut the Fair, he ends up agreeing with Tristan that Yseut the Fair is a more beautiful and worthwhile love interest than his own sister. Yikes, that is cold.
Yseut of the White Hands doesn't really show up that much in the text, but she does get a big finish at the very end of the story. Sick of being dumped on and full of jealousy, she lies to Tristan about the color of the approaching sails. Her lie causes the deaths of both Tristan and Yseut. Maybe we're supposed to feel angry with Yseut for this lie, but we just can't help feeling sorry for her at the same time. And that may be this character's most important purpose. She's a symbol of all the harm and hurt Tristan and Yseut's relationship has caused even as her "treachery" provokes yet more sympathy for the lovers.