The Cleaning Woman
The cleaning woman seems to be a relatively minor character in the novel. In Part Three she comes sweeping in, taking a job that no one else wants. The story doesn't give us much of a description or a back-story, just that she's had experience with the "worst" in life (3.8).
Apparently this explains why she isn't at all afraid of sassing Gregor whenever she peeks in on him. But this humble character gets a huge role in the story when she announces Gregor's death and even, perhaps, has access to what ultimately happens to his body. Since the Samsas shut her up before she can explain, the cleaning lady's story remains untold and a gaping hole is left in the plot. The miracle of getting Gregor out of the apartment without attracting attention is left unexplained.