Proto-heroines cause nothing to actually occur in the plot -- they may, for instance, have outridden every cowpunch in the state in the establishing scenes, and yet get shot promptly as soon as a love-interest is there to be preëminent. Annoying. And, partly because it's difficult to have two of the simpler kind of adventure lead in the same plot, when a hero and heroine marry one of them sort of has to stop... Rochester? reformed rakes in toto? Ekaterina, in the last Barrayar novel; I was annoyed about that.
In contrast, Vimes' subtle wife causes almost everything in this book; she makes the suggestion to Vetinari that puts the plot as a whole in motion, she notices one of Vimes' best clues, she outfaces a king and gets in some gratuitous gratifying violence when locked up. Pratchett has some throwaway, academic-humor lines about the Fifth Elephant as a dwarf's metaphor for the secret pattern, and Lady Sibyl is the Fifth Elephant.
Now, to the dwarves, the Fifth Elephant is also a handy source of schmaltz; and Lady Sibyl also. A coloratura! Really! Such typecasting!
Find in a Library, The Fifth ElephantSo wrote clew in SF&F.