Mrs. Fairfax is first introduced in Chapter 11, which is when Jane arrives at Thornfield and is taken in to its rather grim and dark interior. However, she is pleasantly surprised to see a rather cheery fire and to meet the following personage:
...wherein sat the neatest imaginable little elderly lady, in widow's cap, black silk gown and snowy muslin apron; exactly like what I had fancied mrs. Fairfax only less stately and milder looking. She was occupied in knitting: a large cat sat demurely at her feet; nothing in short was wanting to complete the beau-ideal of domestic comfort.
Mrs. Fairfax is therefore introduced as a very homely and warm woman who relates very well to Jane, and whose opinion Jane respects. She also acts as a kind of guide to Jane during her time at Thornfield, when she has to learn so much. Of course, let us remember that Jane believes her employer is Mrs. Fairfax initially, so she is pleased to learn that she is just a housekeeper.