In Prince Caspian, Lewis states that Reepicheep is descended from a group of common mice who gnawed through the ropes that bound Aslan to the Stone Table after the White Witch had killed him; as a result of their good service, they became talking mice. The mice that freed Aslan from his ropes at the Stone Table might be a reference to when Joseph of Arimathea asked Pilate for the body of Jesus Christ to bury in the tomb.
Reepicheep is depicted as a brave, strong, feisty, and swashbuckling creature. He is extremely courageous, very rarely, if ever, backing down from the face of evil. Despite this, he has a very kind heart, and his devotion to the Light side is beyond irrevocable. He does not tend to feel hatred, despite being very annoyed with people who find him annoying (especially Eustace Scrubb, though the two later get along). He also seems to be annoyed with people who think he is cute, alluding to his dignity. He is very fond of Aslan, leading to his protectiveness of his tail after the latter regrew it for him. This is shown when he challenges Eustace when he refuses to unhand his tail, though he goes easy on him.
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, near Aslan's How. He and his followers run through the battle and plunge their rapiers into unsuspecting enemy's feet, causing them to fall, after which they are 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 "the honor and glory of a mouse." Aslan expresses concern about whether the mouse thinks too much of his honor. However, Peepiceek (the second in command of the mice) and the other mice prepare to cut off their own tails to avoid bearing an honor denied to their leader. This solidarity convinces Aslan to restore Reepicheep's tail, apparently not "for the sake of his dignity," as Aslan states, but for the love Reepicheep's people bear him.
In the 2008 Walt Disney film version, Reepicheep led two other mice into Miraz's castle during a mission that did not appear in the book. The three bound and gagged a cat during the operation. Halfway through, they teamed up with Trumpkin the dwarf and opened the gate, allowing Narnian soldiers into the castle.
In The Voyage of the Dawn Treader, Reepicheep accompanies 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 there because when he was a baby, a dryad prophesied:
Where sky and water meet,
Where the waves grow sweet,
Doubt not, Reepicheep,
To find all you seek,
TThThere is the utter East.
During the journey, Reepicheep teaches Eustace Scrubb about honor, courage, and loyalty, even though at the start of their relationship Eustace treats Reepicheep with great disrespect. They eventually become fast companions, further accentuated in the 2010 film adaptation with the mouse continually riding Eustace in his dragon form in the air.
Near the Eastern edge of the world, Lucy, Edmund, Eustace and Reepicheep set forth from the Dawn Treader in a small rowboat to the edge of the world. The three children are returned to our world through a door in the sky, while Reepicheep embarks in his own small coracle, paddles off in a different direction, and disappears. Lewis then implies, but does not explicitly state, that Reepicheep ultimately did reach Aslan's Country.
When the journey is finished, Reepicheep asks for Aslan's permission to enter Aslan's Country and Aslan says yes. Reepicheep bids farewell to his friends and Caspian before giving up his rapier and before paddling a small canoe up the wave to enter Aslan's Country.