Reepicheep is a Talking Mouse from Narnia, the Chief of his people, and an old friend of Lucy and Edmund from their adventures in the previous book, Prince Caspian. Although he is only about two feet tall (or maybe because he is only two feet tall), his greatest concern in life is honor. For the most part, Reepicheep refuses to let his size limit what he can do. He challenges his enemies and fights on behalf of his friends with stalwart valor, regardless of any potential danger to himself. He dreams of glorious battles, and as he tells Eustace, his burrow at home is filled with books detailing the adventures and mishaps of great heroes.
Reepicheep's preoccupation with courage can often be dangerous to himself and his companions. He is the one who insists that the Dawn Treader sail into the cloud of darkness surrounding the island where dreams come true, and he also leaps overboard to fight the Sea People singlehandedly when one of them shakes a fist at Lucy. However, Reepicheep can occasionally be realistic about his limitations. He knows he's not big enough to take a turn rowing the Dawn Treader, and when the ship encounters a terrible storm, he stays below decks so he won't be swept away.
In The Voyage of the Dawn Treader, we learn something about Reepicheep that was never mentioned in the previous book: when he was a baby, a Dryad pronounced a prophesy over his cradle. The prophecy was in the form of a little song:
Where sky and water meet,
Where the waves grow sweet,
Doubt not, Reepicheep,
To find all you seek,
There is the utter East. (2.22)
Reepicheep explains to Edmund and Lucy that this prophecy has affected him his whole life. Caspian's stated mission for the Dawn Treader is to seek out the seven missing lords who were exiled by his evil uncle, King Miraz. But Reepicheep has a loftier goal: to find the eastern end of the world. He suspects he might be able to sail or travel all the way into Aslan's country (the Narnian equivalent of heaven), and he won't stop until he gets there, or dies trying. His quest is much like a sacred pilgrimage, and when he eventually does make it to Aslan's country, his physical movement into a spiritual place reminds us of the part of the Bible where the prophet Elijah is taken bodily into heaven (in other words, he doesn't have to die first).