Who is fanny? Who or what does she represent in the novel? What does Austen use her to highlight in the novel?

Fanny Price is the poor relation the Bertrams take into their wealthy home to raise. The novel is told almost entirely from her point of view.
Fanny highlights the plight of the poor relation. She represents both the value of a good education, which means not spoiling a child, and the value of integrity.
From the start of the novel, we know that Fanny is not to be treated the same as the four Bertram children. As Sir Thomas says,

There will be some difficulty in our way [...] as to the distinction proper to be made between the girls as they grow up: how to preserve in the minds of my daughters the consciousness of what they are, without making them think too lowly of their cousin; and how, without depressing her spirits too far, to make her remember that she is not a Miss Bertram. [...] they cannot be equals.

Although Mrs. Norris promises to handle this with delicacy, in fact, she treats Fanny very poorly, telling her at one point that

Remember, wherever you are, you must be the lowest and last.

It is no wonder that Fanny, who has no one to advocate for her except Edmund, often retires to her white attic to cry, for she

had known the pains of tyranny, of ridicule, and neglect.

Yet, in the end, this education in humbleness and making herself useful to others, especially to her Aunt Bertram, serves her better than the more privileged upbringing of her cousins. With the exception of Edmund, her cousins grow up to be spoiled, selfish, and self-centered. Maria, particularly, comes to a bad end.
Fanny, on the other hand, who has no fortune and has been raised to think little of herself, attracts a marriage proposal from the wealthy, but wicked, Henry Crawford, something he withheld from Maria despite all his flirting with her. Fanny, however, has the integrity to withstand his advances, and she holds true to her love for Edmund. Despite intense pressure from her uncle and Mrs. Norris, Fanny refuses to marry Henry, knowing he is not trustworthy and putting love and honor ahead of a wealthy establishment.
Fanny's rough education as "lowest and last" wins the day for her. She marries Edmund and earns the esteem of Sir Thomas and Lady Bertram. In the end, although Tom will inherit the estate, she is the true spiritual heir of Mansfield Park, for, like Sir Thomas, it is "the peace and tranquillity of Mansfield" that she most values.