From her first entrance, which startles Mr. Lorry exceedingly, Miss Pross establishes herself as a comic character but also a character that represents loyalty and and love to her "Ladybird", Lucie Manette, of course culminating in her terrific struggle with Madame Defarge towards the end of the novel.
In Chapter 4 of Book I, Miss Pross is described as the character who bursts in when Lucie faints upon hearing the news that her father is still alive:
A wild-looking woman, whom even in his agitation, Mr. Lorry observed to be all of a red colour, and to have red hair, and to be dressed in some extraordinary tight-fitting fashion, and to have on her head a most wonderful bonnet like a Grenadier wooden measure, and good measure too, or a great Stilton cheese, came running into the room in advance of the inn servants, and soon settled the question of his detachment from the poor young lady, by laying a brawny hand upon his chest, and sending him flying back against the nearest wall.
So aggressive is her entrance, in fact, that Mr. Lorry assumes her to be a man, but she quickly shows her compassion to Lucie whilst at the same time ordering around Mr. Lorry ("you in brown") and the inn servants. Her attachment and affection for Lucie continues to be established in the tale, perhaps most amusingly when she assures Mr. Lorry that there are "hundreds of visitors" all paying suit to Lucie, when in fact there are only three. She, with Mr. Lorry, is responsible for the destruction of Dr. Manette's work bench and tools, which again demonstrates her love and care of Lucie's family.
Of course, Miss Pross's love comes into its own as she fights Madame Defarge at the end of the novel to ensure Lucie's escape. Consider how a part of this struggle is related:
Madame Defarge made at the door. Miss Pross, on the instinct of the moment, seized her round the waist in both her arms, and held her tight. It was in vain for Madame Defarge to struggle and strike; Miss Pross, with the vigorous tenacity of love, always so much stronger than hate, clasped her tight, and even lifted her from the floor in the struggle that they had.
Even though she loses her hearing in the struggle, Miss Pross, in her own words, "does not care tuppence" for her life, and willingly shows herself as a loyal loving servant willing to sacrifice herself for those she loves (just as Carton lays down his life for Darnay).