A Thousand Splendid Suns and The Kite Runner are both books by the Afghan American novelist Khaled Hosseini. The Thorn Birds is a novel by the Australian author Colleen McCullough.
To answer a question like this, it is important to consider what the primary traits of a character are. For example, Fariba in A Thousand Splendid Suns is a mother, someone who struggles with depression, and a character who dies tragically. However, I believe that the most defining characteristic of Fariba is that she is unable to overcome her sadness after her sons die. Despite the need to move on with life and serve as a mother for her daughter, Laila, Fariba remains locked in the past and is unable to move past the loss of her sons. This leads Mariam to serve as a surrogate mother for Laila.
In The Kite Runner, the character who struggles the most with moving beyond past trauma is the main character, Amir. When Amir was young he saw the neighborhood bully beat and rape his best friend and servant, Hassan. Amir was too afraid to intervene, and instead of telling his father about what he witnessed, Amir decides to keep it secret. He even goes so far as framing Hassan for theft in an effort to drive Hassan out of the house. It is not until Amir is middle-aged that he is able to atone for his failure to help Hassan. For the intervening years Amir dwells on his actions, and although he marries, he is unable to fully shed the past that haunts him.
In The Thorn Birds, the character most like Fariba and Amir is Mary Carson. Mary Carson is a very wealthy woman who owns a large company as well as a sheep farm that many of the other characters work on. Mary Carson becomes very jealous of the relationship between a local girl named Meggie and the local priest, Ralph. Although Mary Carson had promised her estate to Meggie's father, she changes the will to leave the estate to the Roman Catholic Church with Ralph as the executor. This ensures that Ralph will move far away from Meggie and will break up their relationship. Mary Carson's decision to leave her estate to the church severely impacts Meggie's entire family and causes decades of sadness for Meggie and Ralph.
All three of these characters let their own flaws and inability to let go of past grievances impact their life and the lives of those around them. Through their fixation on the past, they are unable to consider the impact they will have in the present or future, and as such, they cause hurt that reverberates for years. Fariba's neglect of her daughter, Amir's unkindness to Hassan, and Mary Carson's meddlesome will-changing all cause those close to them—Laila, Hassan, and Ralph—to suffer for the rest of their lives.