The characters that change or develop the most in Khaled Hosseini's A Thousand Splendid Suns are Mariam and Laila, the two protagonists who must grow and adapt to cope with their often difficult circumstances.
Mariam is an illegitimate child who grows up in her mother's hut and nearly idolizes her father. However, Mariam learns at a fairly young age that her father will never treat her the same way as he does his legitimate children. In fact, he disappoints her badly on her fifteenth birthday, and not long after, Mariam is forced to marry the abusive (and much older) Rasheed. From there on out, Mariam must learn how to cope with many things. She has to deal with her husband's abuse, her inability to conceive a child, and the arrival of Laila as Rasheed's second wife. Mariam must either develop her strength or die. She chooses the former, so much so that she is able to defend Laila from Rasheed, killing him in the process, and accept her punishment without regret.
Laila, too, develops greatly over the course of the novel. She is an educated girl from a good family, but her mother is cold to her. Laila loves Tariq, but he goes off to fight. Her family faces tragedy when her brothers are killed, and later, her parents die in a rocket attack that almost takes Laila's life as well.
Mariam finds Laila and takes care of her, and then Rasheed marries her. Laila has no choice in the matter, and she has to learn how to get along in a whole new life. Laila becomes pregnant and has a daughter, much to Rasheed's disgust, and from there, Laila and Mariam develop a close relationship. The two women grow together and try to escape from Rasheed, only to be caught. Eventually, Laila has an opportunity for freedom when Mariam kills Rasheed, and she joins Tariq to start a new life.
Indeed, these two women have had to change and develop to meet their circumstances and to survive. They learn and grow together, and only Laila still lives by the end of the novel, but she will always remember the lessons Mariam taught her about courage and love.