Mary is often the forgotten character in Pride and Prejudice; how does the novel define her character?
The story indicates that Mary had always been known in the family for being plain and for the high attitude she takes when she speaks. She is the holier than thou speaker of the family, quite clever admittedly, yet she uses her wits to try to get attention, to her disadvantage that the family is not a very intellectual one. The first to admit to this fact is Mr. Bennet, who always received Mary's comments with a smart remark.
Mary is also smitten by Mr. Collins. When he came to Longbourne he was pretty much a male version of Mary: He spoke too much and said too little, he assumed a mighty high-minded attitude, and he would brag so much about his position in society and that of his patroness that it was enough to impress a girl as plain as Mary.
We know of Mary's admiration in the way that she praised Mr. Collins, and the faint glimmer of hope she displayed when she knew that he was coming to propose to one of the sisters.
Finally, we know that Mary thinks way highly o herself when it comes to the piano and entertainment. We know of the fiasco at the Bingley party when her own father asked her to stop singing and playing the piano in front of everyone. We also know that, of all the sisters, Mary ended up a spinster.
Really, Mary has a lot o depth. You can read some lesson plans here on Enotes that specifically speak of Mary. The fact that she is overlooked by her family might make the reader overlook her as well. Yet, compared to Kitty or example, Mary has enough depth for one to conclude that she definitely has a form of middle child syndrome and that she is opaqued by the dramatic lives of her sisters.
When I think of Mary, I often think of American Idol, and the contestants who think that they are much more talented than they actually are--and no one had the heart to tell them until they were shot down by the judges. At parties, Mary is the one to always offer her services at the piano, playing pieces and singing with no talent and an embarassing lack of social awareness. She has good intentions, that's for sure, but is blisfully and sadly unaware of her own mediocrity.
In addition to her lack of musical talent (and lack of awareness about it,) Mary reads interesting books that develop within her very interesting opinions that she has no shyness in sharing. She states her opinions pointedly and with a haughty confidence; she is so certain of her correctness of opinion that it never crosses her mind to revise her thoughts or keep them to herself. Although she is not prone to silliness like her other sisters, she takes things the opposite direction, and is so ultra-serious all of the time that it is humorous to the reader, and to her family (as long as they are in the privacy of their own home, and not on public display).
Jane Austen describes Mary's character with her usual genius ability to thoroughly and completely characterize people; Through Austen's amazing descriptions, Mary becomes so real that we can see her primly perched at the piano, warbling a song that she truly feels is the most beautiful rendition ever performed.
I hope that those thoughts helped; good luck!