Susan Pevensie is the second child of Mr. and Mrs. Pevensie, and the eldest daughter. She assumes the role of mother when the Pevensie children are sent to live in the country, and is the most practical and 'sensible' of all of the siblings. Susan first enters Narnia with the others while trying to hide from the Professor's housekeeper. She is one of the last of the four Pevensies to enter Narnia, and initially is the most reluctant to embrace and accept it.
She travels with the rest of the group to Mr. Beaver's house, where she along with the others learn about Aslan and the Prophesy. When Edmund turns up missing, the group sets out to travel to Aslan's Camp, and meet Father Christmas along the way. From him Susan receives a Bow and Arrows that do not easily miss, and a Horn, that summons help when blown.
Afterwards they journey to Aslan's Camp and later uses her Horn when the camp is attacked by Maugrim.
Susan accompanies Lucy when Aslan mysteriously leaves the camp, and witnesses his death and resurrection. She then travels with Aslan and Lucy to the White Witch's castle, where she aids Aslan in the wakening of the statues.
After the Battle of Beruna she is crowned, along with her other siblings, ruler of Narnia. She reigns as Queen Susan the Gentle for 15 years.
Around 1014 NT, during the Golden Age of Narnia, Susan receives an offer of marriage from Prince Rabadash of Calormen. She knows him to be a cruel ruler and an even less desirable man, but nonetheless travels to Calormen to visit the city of Tashbaan. Accompanying her are a group of Narnian royals, namely King Edmund, Mr. Tumnus, and Prince Corin of Archenland.
During their visit they realize that their lives will be in danger if Susan refuses Rabadash's offer of marriage, and contrive a plan to leave the city secretly on their ship, the Splendour Hyaline. When they make port at Cair Paravel they receive word that Archenland is under attack, and although many Narnians ride off to aid their neighbors, Susan is not of this group.
Soon afterwards the four Pevensies are hunting the White Stag in the Western Woods of Narnia and accidentally stray back through the Wardrobe. Susan's horn, which she had been carrying with her, is lost.
Susan did not enter Narnia until a year later, when Prince Caspian blew the Horn and summoned the four Pevensies back into Narnia. Arriving at a ruined castle, it is Susan who first discovers that they are actually at the ruins of Cair Paravel, by an old chess piece of hers that she found. Soon afterwards Trumpkin the dwarf shows up, and he and Susan have a contest to see who is the better archer. Later, the group begins its journey to aid Caspian.
Very much reluctant to accept the reality of Narnia for a second time, Susan does not believe Lucy when she claims to have seen Aslan, and when Aslan actually shows up later that night, Susan is among the very last to see him.
Soon afterwards Susan and Lucy travel with Aslan through Narnia, and they assist him in the reviving of the Old Narnia.
After the Second Battle of Beruna, Susan is told, along with Peter, that they are too old, and can never return to Narnia. Blown on Aslan's breath, Susan leaves Narnia for the last time.
Of what became of Susan afterwards not much is known. A year or so after her last visit to Narnia she travels to America with parents; chosen because Mrs. Pevensie thought it would do her 'the most good'.
We know that later, in her desire to be mature and grown-up, she soon forgot all about Narnia, and ceased to be known as a Friend. Consequently, because of all the events that transpired in "The Last Battle", Susan was the only member of her family left living.
As Aslan once said, "No one is told anyone's story but their own". Whether she continued living as she always had, or that she eventually believed in Narnia and went to Aslan's Country when she died, we do not know.