Susan Pevensie is the second child and oldest daughter in the Pevensie family, and changes throughout the Chronicles of Narnia series.
In her early youth she went with her siblings to the house of Professor Kirke to stay safe from the bombings on London. When her sister, Lucy, found her way to Narnia she found herself protecting Lucy from their brother Edmund, though never quite believing her. When she first stepped into the wardrobe she was faithful, staying by Aslan and on the side of good throughout the The Lion, the Witch, and the Wardrobe and became Queen Susan the Gentle.
Later she was courted by the Prince Rabadash of Calormen and lead astray by what he pretended to be. She traveled with her brother, Edmund, Mr. Tumnus, and a few other friends only to find the saying true that you must “See the bear in his own den before you judge of his conditions”. For, she said, “I take you all to witness what marvelous feats he did in that great tournament and hastilude which our brother the High King made for him, and how meekly and courteously he consorted with us the space of seven days. But here, in his own city, he has shown another face.”
With the help of her friends she escaped Tashbaan and the home of the Prince, who if he had been able to may have forced marriage, and made it safely home to Narnia. One day, when they were out on a hunt after many other adventures her siblings and herself once again found their way through Lantern Waste and through the wardrobe to their own world.
A year later in the time of our world she found herself on a platform awaiting a train with her two brothers, Peter and Edmund, and her younger sister, Lucy. There they were taken by the power of Aslan back into Narnia, and there they worked to make sure the rightful king, Caspian the Tenth, was not overtaken by his tyrannical uncle Miraz.
It was then the changes in Susan began showing themselves, when an argument erupted in their group over which path to take, she ignored the fact that she knew she had seen Aslan, and opted for the seemingly easier path which lead to disaster. Then, when Lucy saw Aslan again and woke the others up, Susan was mean to her and insisted the rest not follow Lucy. She later admitted this to Lucy when they were about to face Aslan, saying “I’ve been far worse than you know. I really believed it was him—he, I mean—yesterday. When he warned us not to go down to the fir wood. And I really believed it was him tonight, when you woke us up. I mean, deep down inside. Or I could have, if I’d let myself. But I just wanted to get out of the woods and—and—oh, I don’t know. And what ever am I to say to him?” Aslan showed her his mercy by saying she had listened to her fears, but letting her know he forgave her anyway.
In The Last Battle, the final book in the Chronicles of Narnia series, Susan does not accompany her siblings to Aslan’s Country, for she had turned away from the truth that she had known.
“Sir” said Tirian, when he had greeted all these. “If I have read the chronicle aright, there should be another. Has not your Majesty two sisters? Where is Queen Susan?”
“My sister Susan,” answered Peter shortly and gravely, “is no longer a friend of Narnia.”
“Yes,” said Eustace, “and whenever you’ve tried to get her to come and talk about Narnia or do anything about Narnia, she says, ‘What wonderful memories you have! Fancy your still thinking about all those funny games we used to play when we were children.'”
“Oh, Susan!” said Jill. “She’s interested in nothing nowadays except nylons and lipstick and invitations. She always was a jolly sight too keen on being grown-up.”
“Grown-up, indeed,” said the Lady Polly. “I wish she would grow up. She wasted all her school time wanting to be the age she is now, and she’ll waste all the rest of her life trying to stay that age. Her whole idea is to race on to the silliest time of one’s life as quick as she can and then stop there as long as she can.”
– The Last BattleChapter VII : Through the Stable Door
Susan did not die with her siblings, and so it is unknown if she ever changed and again became a friend of Narnia. In her later life she may have again turned to the ways of her childhood, and truly grown up. The truth is only known by the creator.