We have two big ideas to discuss when thinking about Sabina: kitsch, and betrayal. Before we start, you should go ahead and read what we have to say about kitsch in "Symbolism, Imagery, Allegory." We talk there about the origin and definitions of the word, as well as Kundera's unique interpretation of the concept.
Talking about kitsch leads us right into our second Sabina-centric topic: betrayal. We've seen that Sabina declares kitsch her enemy, and now we'll see that betrayal is Sabina's way of fighting kitsch. That's worth saying a second time: betrayal is Sabina's way of fighting kitsch. In "Symbolism, Imagery, Allegory," we saw that the enemies of kitsch were beauty and individuality. Through betrayal, Sabina achieves both of these.
We'll start with beauty. "Beauty is a world betrayed," thinks Sabina. "The only way we can encounter it is if its persecutors have overlooked it somewhere. Beauty hides behind the scenes of the May Day parade. If we want to find it, we must demolish the scenery" (3.7.14). Her paintings are their own form of betrayal because through them Sabina rejects the aesthetic ideology imposed on her by the communist regime. At the Academy of Fine Arts, her professor of Marxism "expounded on the theory of socialist art" and claimed that "Soviet society had made such progress that the basic conflict was no longer between good and evil but between good and better" (6.10.1).
Sabina's response was a giant aesthetic betrayal; she betrayed her schooling by painting what lurks behind the May Day parades. "On the surface, there was always an impeccably realistic world," she explains to Tereza, "but underneath, behind the backdrop's cracked canvas, lurked something different, something mysterious or abstract" (2.20.4). "Of course," she adds, "I couldn't show them to anybody. I'd have been kicked out of the Academy" (2.20.4). This is where the line "On the surface, an intelligible lie; underneath, the unintelligible truth" comes in, a phrase we'll see repeated many more times in the course of the novel (yet another example of Kundera's strategic repetition, as discussed in "Writing Style").
Now on to individuality. Remember Sabina's bowler hat? The narrator tells us that "it was a sign of her originality, which she consciously cultivated" (3.2.8). In Sabina's mind, betrayal is about asserting your individuality against the conformity imposed by totalitarian kitsch. "Betrayal means breaking ranks," the narrator explains (3.3.12). "Betrayal means breaking ranks and going off into the unknown. Sabina knew of nothing more magnificent than going off into the unknown" (3.3.12).
Sabina's career as an artist is a fundamental a part of her character – not unlike the way Tomas's career as a surgeon defines who he is. This may just be another example of es muss sein ("it must be").
For at least the first half of the novel, you might be inclined to describe Sabina as a woman who embraces lightness – especially if you associate "lightness" with sexual promiscuity and "weight" with the sort of commitment that a woman like Tereza brings with her. But as we've seen time and time again, slapping these labels on the characters is not the way to go. Sabina might appear to revel in the sweet lightness of being, but let's not forget Kundera's central thesis here: the lightness of being is unbearable.
Now, if you think Sabina is the exception to the rule, go ahead and find the only three passages in the novel, aside from the title, where the phrase "unbearable lightness of being" is actually used. Or just read them here:
When we want to give expression to a dramatic situation in our lives, we tend to use metaphors of heaviness. We say that something has become a great burden to us. We either bear the burden or fail and go down with it, we struggle with it, win or lose. And Sabina-what had come over her? Nothing. She had left a man because she felt like leaving him. Had he persecuted her? Had he tried to take revenge on her? No. Her drama was a drama not of heaviness but of lightness. What fell to her lot was not the burden but the unbearable lightness of being. (3.10.2)
The thing that gives our every move its meaning is always totally unknown to us. Sabina was unaware of the goal that lay behind her longing to betray. The unbearable lightness of being-was that the goal? Her departure from Geneva brought her considerably closer to it. (3.10.5)
She was well aware it was an illusion. Her days with the aging couple were merely a brief interval. […] Sabina's path of betrayals would then continue elsewhere, and from the depths of her being, a silly mawkish song about two shining windows and the happy family living behind them would occasionally make its way into the unbearable lightness of being. (6.12.8)
Notice that all three are about Sabina.
Now we won't argue with you that Sabina pursues lightness – no doubt about that. As we see in the second passage above, the narrator speculates that this is indeed "the goal that [lies] behind her longing to betray." And at the end of the novel, her wish to be cremated and scattered to the winds is right on par with the pursuit of lightness. "Tereza and Tomas had died under the sign of weight," explains the narrator, and so Sabina "wanted to die under the sign of lightness" (6.25.4). But just as we saw with Tomas's attempt to cast off his es muss sein and become a window washer, the pursuit of lightness doesn't make the experience of lightness any less unbearable.