If you've read Pnin's Character Analysis, you already know that Liza Wind is not an ex-wife you'd wish on your worst ex-friend. She married him out of spite when one of her suitors rejected her, then had an extramarital affair that resulted in a son, tricked Pnin into paying for prenatal care and passage to America, and basically made him pay child support for a kid he has never met. Oh, and she went on to marry a third guy after kicking the second guy to the curb. A charming lady, no?
And after all of that, Pnin still loves her. As if his awkwardness weren't enough, he's got to go and stay in love with a demoness. Honestly, she doesn't deserve him and his bumbling ways. But let's not be too hard on Liza. The way that the narrator describes her seems to imply that she just can't help it.
The narrator says:
Actually her eyes were of a light transparent blue with contrasting black lashes and bright pink canthus, and they slightly stretched up templeward, where a set of feline little lines fanned out from each. She had a sweep of dark brown hair above a lustrous forehead, and a snow- and-rose complexion, and she used a very light red lipstick, and save for a certain thickness of ankle and wrist, there was hardly a flaw to her fullblown, animated, elemental, not particularly well-groomed beauty.(2.5.1)
This isn't just saying that she's pretty. The words that the narrator uses are intentional. She has a "set of feline little lines," and an "elemental, not particularly well-groomed" beauty. In other words, she's got a little chunk of animal in her but she's natural in a way that means she's not out for blood.
Basically, this all seems to imply that Liza isn't really all that crafty. She just can't help herself. Kind of like a kid who begs for candy. By comparing her to an animal, or something elemental, the narrator implies that she isn't meticulously planned or made up. Animals are generally not thought to be able to plan like humans, and saying that a person is animalistic means that they are instinctual. So just like a lion or a tornado can cause death but not be technically cruel, Liza just doesn't seem to know better.
And maybe that's exactly what Pnin likes about her. The narrator says: "He saw her off, and walked back through the park. To hold her, to keep her—just as she was—with her cruelty, with her vulgarity, with her blinding blue eyes, with her miserable poetry, with her fat feet, with her impure, dry, sordid, infantile soul" (2.6.37).
Pretty intense, huh? If we are to believe VN's description of Pnin's thoughts (which is a whole different story), then we see that Pnin is fully aware of what Liza really is like, and still feels some kind of attraction to her.