When Trystram first meets Isode, she is "the fayrest lady and maydyn of the world" (238.4) and her name, "La Beale Isode," French for "the beautiful Isode," tells us as much. She's also a skillful healer, leeching Trystram's wound of poison so that he comes back from the brink of death. All in all, she seems like a pretty good catch, right?
Other than these two attributes, though, we don't really learn that much about Isode's character. Like many women in medieval romance, she's a mostly just a pawn in the schemes and battles of men. Although she declares her love and devotion to Trystram, she must submit when her father "gives" her to Trystram who, in turn, gives her to Mark. She has no say in the matter whatsoever. Likewise, she has no choice but to allow Sir Palomydes to kidnap her when Mark commands her to do so.
Still, no matter how much she is subject to the whims of men, Isode definitely has a mind of her own. We get to see that in play during a later episode with Sir Palomydes. Isode observes Palomydes' incognito defection from Trystram during a tournament, and calls him out for it in front of them both, asking, "how sholde I suffir in youre presence such a felonne and traytoure as ys Sir Palomydes?" (446.29-30). You go girl.
Or… not. Despite her outburst, Trystram has already decided to believe Sir Palomydes' version of events, and commands Isode to let the matter go. Isode is forced to submit, as she always is. At least we get a brief glimpse of her will power, even if it's defeated in the next moment.
Yet in one important way, Isode stands up to the powerful men around her. She has an affair with Trystram, her true love, rather than remaining faithful to her husband, King Mark. While that might seem a bit less-than moral, it also gives us another way to look at female adultery in Camelot. As it turns out, cheating on your husband with your true love is one of the few ways a woman can take control over her life. It's a questionable moral choice, of course, but at least it's a choice.