"That's all right," said Edmund. "Between ourselves, you haven't been as bad as I was on my first trip to Narnia. You were only an ass, but I was a traitor."
-- Edmund Pevensie to Eustace Scrubb, The Voyage of the "Dawn Treader"
A character appearing in five of C.S. Lewis' Chronicles of Narnia: The Lion, the Witch, and the Wardrobe, Prince Caspian, The Voyage of the "Dawn Treader", The Horse and his Boy, and The Last Battle.
Edmund was a rude, spoiled child who enjoyed tormenting his younger sister Lucy. They and their older siblings Peter and Susan lived in Professor Digory Kirke's great old country house during the London air raids. When Lucy discovered a gateway to the world of Narnia in an old wardrobe, Edmund disbelieved and mocked her. Later he followed her to Narnia, where he met Jadis, the White Witch who ruled Narnia. After hearing his story (plying him with enchanted Turkish Delight), and remembering the prophecy that when four humans sat on the thrones of Cair Paravel her reign and life would end, Jadis convinced Edmund to betray the others to her. All four children later journeyed to Narnia, and Edmund set out to do exactly that.
He made his way to her castle, where she (predictably) enslaved him and sent her wolves to kill the others. Peter, Susan and Lucy escaped, however, and found their way to the Witch's archenemy Aslan the great Lion. Aslan sent his forces to rescue Edmund, who repented of his misdeeds.
That was not enough; an ancient Narnian compact gave the Witch the right to a traitor's life. Aslan laid down his own life for Edmund's instead -- but was resurrected after his sacrifice. In the final battle at the Fords of Beruna, Edmund was gravely wounded, but not before destroying the Witch's magic wand. He was healed by Lucy's magic cordial.
Edmund and his siblings were named Kings and Queens of Narnia, and ruled for many years until they blundered back into our world (returned to their "proper" ages).
A year later, Edmund and the others returned to Narnia a thousand years in its future, having been summoned to help Prince Caspian take the rightful throne from his uncle King Miraz the Usurper. Still another year later, he, Lucy and Eustace Scrubb accompanied Caspian on the Dawn Treader's voyage to the World's End. Aslan met them and said that this would be Edmund and Lucy's last trip to Narnia.
Finally, an apparition of King Tirian of Narnia appeared before Edmund and six other Friends of Narnia at a dinner party many years later. Peter and Edmund disguised themselves as workers and retrieved the Magic Rings which had facilitated the first journey of humans to Narnia. As they were returning the rings a devastating rail accident occurred, and Edmund found himself alongside the other Friends in a truer, better Narnia -- his final reward for a lifetime of service to Aslan.