From the beginning of Pride and Prejudice we see the character of Mary depicted in a different light than that the rest of the sisters. However, there is much more to Mary than meets the eye.
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.