In Chapter 16 the reader discovers more about the character of Agnes and in particular certain aspects of her character that make her very suitable to be David's future wife. Firstly, both have suffered the loss of a parent at an early age. For Agnes, it was her mother who died, and for David, his father. As a result, both are (or were, in David's case) completely devoted to their remaining parent.
In addition, Agnes shows that she is very intellectually gifted, which distinguishes her from Dora. Note what David says about her talents:
In good time she made tea; and afterwards, when I brought down my books, looked into them, and showed me what she knew of them (which was no slight matter, though she said it was), and what was the best way to learn and understand them. I see her, with her modest, orderly, placid manner, and I hear her beautiful calm voice, as I write these words.
Agnes is similar to David because of her intellectual abilities. The fact that she is able to talk seriously with David about what he is learning and to offer him advice on the best way to proceed in his studies clearly indicates that she is similar to David in this regard. They are more intellectual equals, and this is something that is very important when Dora is compared to Agnes. One of the central themes of this novel is what makes a suitable marriage. Dickens points towards the unavoidable conclusion that a certain equality of temperament and mind is essential.