In The Lion, the Witch and the Wardrobe, Edmund is one of the main characters, at the age of 10-years-old, and the character who develops the most over the course of story.
It is implied in the book that Edmund started life as a likeable person, but then changed for the worse and began to act meanly after attending a new school. However, in the 2005 film adaptation of the book, it is implied that he is upset that their father was forced to serve in the war and that they are sent away from home to the countryside in order to avoid the Blitz. In the 1988 BBC version, the reason for his change in behavior is not mentioned.
Edmund makes sarcastic comments to Lucy when she first finds the entrance to Narnia through the wardrobe, and is the second of the Pevensie children to go to Narnia, after following Lucy to tease her. While there, he meets the White Witch (who introduces herself as the Queen of Narnia) and eats some enchanted Turkish Delight, which causes an addiction in the person who eats it. As a result, he promises the Witch that he will bring his siblings to her house, not knowing that she intends to kill them all to prevent the fulfillment of a Narnian prophecy. Lucy did mention the White Witch in a subsequent conversation and Edmund realised that the witch was none other than the "Queen of Narnia", but the magic of the Turkish Delight was so strong that he was determined to go back to her for more.
Upon returning, he denies having been in Narnia, not wishing to admit that Lucy's story had been true.
It is when all four of the Pevensie children later go through the wardrobe that he lets slip that he has been in Narnia before. He and the other three children are taken under protection of Mr. and Mrs. Beaver, but while the others are having an in-depth conversation about the arrival of Aslan, Edmund sneaks away to the White Witch's castle, where he expects to be made a prince and later a king.
However, his opinion of the Witch changes dramatically when she berates him for coming alone, and even more so when on their journey to the Stone Table, they encounter a group of creatures enjoying a feast provided by Father Christmas. When the creatures continue to affirm that Father Christmas is their benefactor and has entered the land, a clear sign of her waning power, she turns them to stone over the protests of Edmund. (In the 2005 film after Maugrim catches the fox which helped the beavers and the other three Pevensies elude him, the witch turns the fox to stone and hits Edmund for withholding information about Aslan and his army. In the 1988 BBC version, the same scenario is shown as in the book.)
He now realizes to his horror the evil with which he has allied himself, and would give anything to be with the others. The sledge eventually stalls as the snow melts (another sign of the witch's crumbling power), so they have to continue their journey on foot. They eventually stop in a wooded valley, where the Witch prepares to put him to death as a traitor. She ties Edmund to a tree and draws her knife, but a rescue party sent by Aslan arrives, frees him, and brings him to his siblings and the rest of Aslan's army. Edmund becomes fully reformed after a long conversation with Aslan who afterward commands the Pevensies to consider the matter of their brother's misdeeds resolved. The next day, the Witch reiterates her claim to Edmund's life. She and Aslan work out an agreement that Aslan will die in Edmund's place (though the other Narnians do not know this), but unknown to her, the magical nature of this contract allows Aslan to be brought back to life. Susan and Lucy witnessed Aslan's sacrifice and resurrection.
While Aslan and Edmund's sisters race to free the cursed prisoners in the Witch's castle, Edmund consolidates his reformation by aligning himself with Peter's army in battle, where he plays a critical role in neutralizing the White Witch's most dangerous advantage, her wand, and is gravely wounded in the attempt. This sees the Witch's army vastly outnumbered very quickly, and she is soon killed by Aslan, while the remnants of the enemy either give themselves up or take to flight.
However, he is saved from death by the timely arrival of reinforcements led by Aslan, and by Lucy, who gives Edmund a dose of a magic cordial, which can quickly heal any injury.
Eventually, a completely reformed Edmund Pevensie is crowned to the Great Western Wood by Aslan as King Edmund the Just, co-ruler of Narnia with Queen Lucy, Queen Susan and High King Peter, and is knighted as Duke of Lantern Waste, Count of the Western March, and Knight of the Noble Order of the Table.
After fifteen years in Narnia, he and his siblings return to England, where they all magically appear as children again.
Edmund and his siblings return to Narnia to aid Caspian, rightful King of Narnia, against King Miraz the Usurper. He convinces Trumpkin the dwarf that they are the Kings and Queens from the legend by defeating him in a sparring practice. He later helps Peter and Trumpkin defend Caspian against Nikabrik, the hag, and the were-wolf. Edmund is also there to witness Peter's duel against Miraz.
He has since become more caring and protective of Lucy, and is the first person to believe her when she sees Aslan, supporting her against the disbelief of Trumpkin and her other siblings. Edmund is shown in a more positive light in this book than in The Lion, The Witch and The Wardrobe. His demeanor is more cooperative and loyal, even under the guidance of Peter, who is unprepared for the new Narnia and its current status.
In the movie, Edmund proves to be much more mature than Peter and Caspian both, but he stays out of their arguments. Also in the movie, he is able to stop the White Witch from being brought back from the dead by forcing his sword into the ice wall she is stuck in, smashing it.
The Voyage of the Dawn Treader
Edmund, Lucy and their cousin Eustace enter the world of Narnia through a magic painting, and end up in the ocean. They are rescued and brought on board the Dawn Treader, where they are reunited with King Caspian, who is on a quest to search for the missing Lords that his uncle had sent to explore other lands some years before. This is Edmund and Lucy's last adventure in the world of Narnia, since Aslan told them they were getting too old to come back. By this point Edmund's character has matured a great deal which can be seen in the way he deals with his cousin Eustace and in the power struggle with Caspian. By this time also, Edmund has completely refrained from mistreating and insulting Lucy. When Eustace changes his behavior after being changed back from a dragon, Edmund mentions his own betrayal and says that Eustace wasn't worse than he was.