A good answer to your question is to list one characteristic of Melinda in the first school quarter and one of her traits from the last quarter. During the first and second quarters of the school year, Melinda is certainly reclusive. She has withdrawn from others, even her own family. This characteristic is a direct result of Melinda's rape. The author infers that before the attack, Melinda was quite social--she has several "good" friends and is excited about attending social events like the party.
Throughout the entire book, but especially during the last quarter, Melinda is witty and sarcastic. I think that what draws many readers to her character is her ironic sense of humor and her witty observations of her teachers and peers. These characteristics are partly what helps Melinda survive and eventually overcome her rape and ensuing isolation.
In many respects, the answer to this is dependent on how the reader perceives Melinda throughout the course of the novel. I have my own feelings about the protagonist, but I think you will have to develop your own understanding of her and the characteristics or traits that help to define her. Perhaps, you could compare how Melinda was before the attack and then afterwards. In noticing the difference, this might provide some characteristics for you. Another scenario where her characteristics develop would be in how she views her peers after the attack. What is she like? In what does she believe? How does she perceive her social order? Answering these questions might give you some other characteristics or traits about Melinda.