Reepicheep is the Chief Mouse. He is the self-appointed servant to Prince Caspian, and perhaps the most valiant knight in all of Narnia. His chivalry is unsurpassed, as also are his courage and skill with the sword. He is honorable and courageous, not to mention lethal with his rapier. His foes may exclaim surprise but these are often their last words. Reepicheep appears in Prince Caspian, The Voyage of the Dawn Treader, and The Last Battle.
In Prince Caspian, Reepicheep is the leader of the twelve mice who help to fight against Caspian's Uncle Miraz in the Second Battle of Beruna at Aslan's How. He and his followers run through the battle and plunge their rapiers into an unsuspecting enemy's foot, causing him to fall, after which he is quickly dispatched. Reepicheep is badly wounded in the battle, and he loses his tail. He petitions Aslan afterward to have it restored, as the tail is his honour. Aslan expresses concern on whether the mouse thinks too much of his honour. However, Peepiceek (the second in command of the mice) and the other mice prepare to cut off their own tails to avoid bearing an honour denied to their leader. This solidarity convinces Aslan to restore Reepicheep's tail.
In The Voyage of the Dawn Treader, Reepicheep accompanies King Caspian on his voyage in the hope of finding Aslan's Country at the end of the world. He believes it is possible to sail to Aslan's Country because when he was a baby mouse, a dryad prophesied that he would find all he sought "...Where the waves grow sweet...There is utter east." During the journey, Reepicheep teaches Eustace Scrubb about honour, courage, and loyalty, even though at the start of their relationship Eustace treats Reepicheep with great disrespect. Near the Eastern edge of the world, Lucy, Edmund, Eustace, and Reepicheep set forth from Dawn Treader in a small row boat to the edge of the world. The three children are returned to our world through a door in the sky, while Reepicheep debarks in his own small coracle, paddles off in a different direction, and disappears. C.S. Lewis then implies, but does not explicitly state, that Reepicheep ultimately did reach Aslan's Country.
In The Last Battle Reepicheep appears at the very end to greet the last of the Narnians at the Emperor's Garden. He is exactly as the Pevensies remember him, not having aged or died in the hundreds of Narnian years that have passed since their time.