Yseut's full title, "Yseut the Fair," should tell you a little something about her. Yup: she's a knock-out. She's so drop-dead gorgeous, in fact, that even the brother of Tristan's wife agrees that she's a hotter and more worthwhile love interest for Tristan than his own sister. But Yseut's not just a pretty face: she's also a skilled healer, and she's got some smarts, too. On several occasions, she's the only one in the world who has the ability to heal Tristan's poisoned wounds. As Tristan says, "She could soon cure my sickness just by calling me her love" (18.159).
Yseut wriggles out of a lot of tight spots using her cleverness. For example, when she notices Mark eavesdropping on her and Tristan, she immediately comes up with a deception to convince him they are not lovers, finishing off with a truth that's not quite true, at least as Mark would understand it: "But before God I swear I have been loyal: may He scourge me if anyone has ever had my love except the man who had me as a maiden" (2.48).
She also arranges to have Tristan dress up as a leper and carry her across a bridge so that she can honestly swear that she's had no man between her legs but Mark and the leper. In fact, on every occasion when Tristan and Yseut successfully avoid exposure, it's Yseut's quick thinking that saves the day.
Yseut is set apart from other women in this story's world not just because of her cleverness but also because of her connection with magic and healing. As far as we know, only Yseut's mother can compete with her in this realm. Just as Tristan seems destined for (unhappy) love because of his connection with love and death, Yseut's connection with magic and healing makes her seem destined for it, too. The love potion is a family recipe, after all. It's almost as if in this story, people who are more connected to magic and the irrational are more likely to experience the kind of all-consuming passion the love potion causes.
Because The Romance of Tristan is totally focused on the relationship between its main characters, it spends a lot more time than most other romances on a female character. Yseut is definitely strong, but despite all her chutzpah and cleverness, she's still subject to the rules of her society. Because she is a woman she will always be a man's possession, to be passed among them at will.
When Tristan defeats the dragon, Yseut's father gives her to Tristan, who promptly declares that he's going to give her to Mark. Nobody bothers asking Yseut, because her opinion doesn't matter: she's just property. Yseut rebels against this status, however, when she takes Tristan as her lover. Granted, she claims that the love potion made her do it, but by allowing a man who's not her husband to possess her body, she claims a small degree of authority over that body.
Like Tristan, Yseut sometimes does things that can make it difficult to totally like her. Like the time she decides to kill off Brangain the scatterbrain. Look, Yseut, we know Brangain pulled a major fail with the love potion. Got it. But she just took your place in bed with the king, she's your friend and loyal servant, and she's got your back, so what's with the execution orders?
Well, hey, sometimes a princess just has to be a princess. We're probably supposed to understand that Yseut was just freaking out: "Yseut realized that Brangain constituted a potential danger, since she alone could betray the lovers to Mark. For her own safety, Yseut decided to have Brangain killed by two of her servants" (1.45). That is a pretty scary situation. And most importantly, she let her friend live, after all. Hey, maybe she's just got an Irish temper.