It's really beside the point to debate who the real protagonist is of Unbearable Lightness. The novel is a montage of the intersecting lives of all four individuals (Tomas, Tereza, Sabina, and Franz). You might argue that we spend more time with Tomas and Tereza, or that their story is more central while Franz and Sabina's is ancillary. Making this kind of claim, however, doesn't help you analyze, discuss, or understand the novel. It's more interesting to look at the way in which the characters intersect through common themes, ideas, phrases, or symbols (and there's plenty of that in this Shmoop guide to go around).