Character Analysis
A Modern Woman
Eva may be an absolute beginner when she first encounters Karel—that striptease to classical music says it all; hopefully it at least wasn't something like the Ride of the Valkyries—but she can talk the talk. She knows what she wants, even though she may blush when she first says it. Kundera describes her in this way:
Eva is a cheerful man-chaser. But she doesn't chase them to marry them. Not love but only friendship and sensuality exist for her. So she has many friends: men are not afraid she wants to marry them, and women have no fear she is seeking to deprive them of a husband. (II.3.4)
Basically, Eva wants to live her life the way she perceives that men like Karel do. And she's utterly practical about it; she has no interest in breaking apart Karel and Marketa's marriage. In fact, she's a huge supporter of their union. She's only in it for the fun, so she doesn't want Karel abandoning his lovely wife for her.
A Woman's Woman
Eva is a beautiful woman for sure. But she's also attractive in a general way. Marketa can't help but be drawn to her the first time they meet in the sauna, and although Eva is kind of a "honey trap"—Karel has sent her to woo Marketa into friendship so that they can have comfortable threesomes—Marketa genuinely appreciates her forthright personality:
...she didn't waste time starting the conversation with conventional remarks...but instead began right away to talk about herself, a bit like people becoming acquainted through a personal ad trying hard to compress into the very first letter to their future partner who and what they are. (II.3.3)
It's little wonder, then, that Eva and Marketa feel more affection and attraction for each other than they do for the self-centered Karel.
A Peacemaker (Sort Of)
Karel needs Eva so that he can indulge his sexual preferences without feeling guilty about Marketa getting left out. Marketa needs Eva so that she can block out her painful past with her philandering husband. So what does Eva get out of this whole deal?
Friendship. And sensuality. Eva is only into it for the pleasure—or so she says. When Marketa and Karel begin to quarrel, Eva springs into action: it's her job to keep the peace so that she can continue enjoying both of them with no guilt.
But has Eva actually met her match? When Karel gets carried away in his pervy fantasy, she's essentially left alone with Marketa in bed—and the two ladies are doing just fine without him. In fact, Eva wants Marketa to visit her next time. Karel isn't invited.
That Male Gaze
It's really easy for us to like Eva as a character. She's charismatic, she's a little awkward, and she seems to care very much for the people in her threesome.
But it's hard to get a read on Kundera's assessment of her. Behind the admiration and sympathy is just a little bit of laughter, as though the author thinks that Eva's forward-thinking ideas about sexuality are just super cute:
...she explained that social conventions meant nothing to her when she was attracted to a man. That she allowed only friendship and sensuality. Signs of embarrassment and effort showed on her face, and Karel felt a kind of fraternal compassion for her rather than desire. (II.5.2)
Kundera is not exactly making fun of Eva here, but he is making her a little bit ridiculous. It's exactly the kind of behavior that Jan accuses women of when they "gaze badly" and spoil things for men. No double standard there at all, Kundera.