Susan Barton is the main voice of the text, functioning as protagonist and narrator for much of the novel.
"My name is Susan Barton, and I am a woman alone."
In the context of Coetzee's oeuvre as a post-modernist and feminist writer, Barton's authority is important and importantly problematic. She is a marginalized and historically silenced figure claiming an authorial voice for herself in the space of a well-known story. She is a question being posed about canonical bias. Bitter and assertive, Susan Barton is also pleading and elegiac, forced to defend the validity of her narrative and her role therein, finally made to proclaim such a basic fact.
"I am substantial; and you too are substantial, no less and no more than any of us. We are all alive, we are all substantial, we are in the same world."
As a figure then, Barton is an assertion. As a character she is a complex assortment of pride and indignation, capacity and reactionary emotion. She has had everything taken from her except the potential to tell her own story and this last power is the very thing she seeks to give away.
Met with silence from Friday and Foe and presented with a man in Cruso who "no longer knew for sure what was truth, what fancy," Susan Barton performs a highly self-reflective and self-questioning narrative that hinges on an examination of social isolation.
Nonetheless, it is through Barton's eyes that we receive characterizations of Cruso and Friday.
Barton describes Cruso as a quiet, middle-aged Caucasian man who has no desire to be rescued from the island. He spends his time building stone walls, laying the groundwork for a terraced farm though he has no seeds to plant for a crop. Friday is an even quieter figure, a slave (or former slave) who has no tongue and who serves as Cruso’s servant, never uttering a word. (eNotes)
Barton herself possesses a sense of privilege that allows for indignity when she is initially shipwrecked. She is distressed at being treated roughly in the mutiny that led her to be abandoned and set adrift on a raft at sea. This is remarkable because someone of a different sensibility may have reacted with a fear for her life instead of a fear for her pride.
Barton does have good reason to feel ill-treated by the world, however. In her description of herself, she outlines her background as a victim of circumstance.
"Two years ago my only daughter was abducted and conveyed to the New World by an Englishman [...] I followed in search of her. Arriving in Bahia, I was met with denials and, when I persisted, with rudeness and threats."
Barton is a woman beset with difficulties and, as the story opens, stranded on an island with a silent man and one who she thinks of as a "kindly man" even while she questions the integrity of his sanity.
In Barton there is a complex but recognizable combination of self-pity and pride that brings her to bring her request to Mr. Foe as if it is also a demand. She is the "author" of much of the novel, yet she is also attempting to put her story into this man's hands so that it can be turned into money. In doing so, the authority over her story is made problematic along with her relationship to the telling and content of that story.
Notably, a considerable portion of her story is taken up by Cruso, over whose life-story Barton takes defacto authorship. Friday too is subject to the literal authority of Cruso then of Barton.
"Friday has no command of words and therefore no defence against being re-shaped day by day in conformity with the desires of others. I say he is a cannibal and he becomes a cannibal; I say he is a laundryman and he becomes a laundryman."
Thus Friday's silence is arguably his most important trait. Malleable and passive, Friday is above all a screen on which others can project their vision(s).
Importantly, Barton is central to Foe but she is not included in Defoe's Robinson Crusoe. Her existence as a mediator between the writer (Mr. Foe) and the actual story of a shipwrecked life on an island is fundamentally important especially as the nature of her role in this regard is perhaps the central issue of Coetzee's novel.