Who gives the reader the first description of Mr. Rochester in these chapters, and how does it affect the story?

The first description Jane and the reader receive of Mr. Rochester is given by Mrs. Fairfax in chapter 11.
Mrs. Fairfax's initial remark that his visits at home were "rare, sudden and unexpected," and that it "put him out to find everything swathed up" leaves Jane with a newfound curiosity to know more about her mysterious and temperamental employer. She asks Mrs. Fairfax numerous questions about the man: Is he exacting and fastidious? Is he generally liked? What is his character? Unfortunately, Mrs. Fairfax turns out to be, as Jane afterwards observes, one of those "people who have no notion of sketching a character," and Jane learns next to nothing about Mr. Rochester from her.
However, it is perhaps Mrs. Fairfax's unintentional evasiveness that serves to influence Jane's subsequent curiosity in her employer's true character, an interest that strongly affects her later actions and therefore the remainder of the story.
For instance, Mrs. Fairfax notes vaguely that her master has "traveled a great deal, and seen a great deal of the world" and that "he is clever." For Jane, who has an unquenchable thirst for learning and who has always dreamed going to far-off lands, this is intriguing to say the least. Jane herself is extremely intelligent, and though she is already growing fond of Mrs. Fairfax and Adele at this point in the book, they are not able to stimulate her intellectually, and she yearns for conversation with someone who can.
Further, Mrs. Fairfax admits that "you cannot be always sure whether he is in jest or earnest, whether he is pleased or the contrary; you don't thoroughly understand him, in short." An introvert with strong intuitive skills, Jane makes a study out of other people's true characters. To come across such a seemingly contrary and puzzling personality as Mr. Rochester's must have aroused her curiosity and surely affected the amount of interest she later showed in him.