Trumpkin is a red dwarf who first meets Caspian after the poor prince has knocked his noggin in the woods. Trumpkin, along with Trufflehunter and Nikabrik, introduces Caspian to the other Old Narnians in hiding, and when it's determined that they'll war with Miraz, he joins the fray.
When Caspian blows Susan's horn, Trumpkin goes to Cair Paravel to see if help truly arrives. There, he finds the Pevensie children, and together they make the treacherous journey back to Caspian's camp. He also joins the fight to stop Nikabrik from resurrecting the White Witch as well as the final battle. So Trumpkin just might be the busiest guy during the events of Prince Caspian. But he's also—
Yep, Trumpkin is to Prince Caspian what Edmund was to The Lion, the Witch, and the Wardrobe. These guys are the "O ye of little faith" characters that don't believe in Aslan until they see him face to face. The dwarf even boldly asks, "But who believes in Aslan nowadays?" (6.69). Not Trumpkin, that's for sure.
But do you remember that scene in The Lion when Aslan showed up and Edmund was totally embarrassed for being such a dink? Well, just like Edmund, Trumpkin's character must have a moment of confrontation when he has to deal with evidence counter to his skeptical ways.
When the Pevensie children finally encounter Aslan, the Lion calls Trumpkin forward. Although the children could "see that Aslan liked the Dwarf very much [and] were not disturbed […], it was quite another thing for Trumpkin, who had never seen a lion before, let alone this Lion" (11.47). It's definitely a knee-trembling experience for the guy.
But seeing is believing for Trumpkin in the end. After Aslan roughhouses with him a bit, the lion asks, "'Son of Earth, shall we be friends?'" (11.49). The dwarf (breathlessly) accepts Aslan's offer of friendship and becomes a true believer.
With New and Improved Features
But Trumpkin isn't an exact copy of Edmund in The Lion. Edmund, if you'll recall, was a foul, snooty little boy who liked to pick on Lucy to get his kicks. He also didn't follow orders very well and generally did as he pleased—in the process, pleasing no one but himself. Only after a harrowing experience with the White Witch and Aslan's sacrifice did he come to his senses and become Edmund the Just.
Trumpkin, on the other hand, is only characterized with the flaw of skepticism and lacks the other rotten parts of a younger Edmund. He is loyal, fair, and follows orders. He follows Caspian's orders to go to Cair Paravel without question—even though he thinks it's a bunch of "'foolery'" and fairytale mumbo-jumbo (7.52). He even reminds Susan of her own duty to "'[o]bey the High King, your Majesty, Peter'" (11.13). In other words, minus that streak of skepticism, which cures itself by way of lion's roar anyway, Trumpkin is one of the novel's positive role models in the themes of "Courage" and "Principles."
Now if only we can work on that cussing mouth of his. "'Horns and halibuts'" (5.48)?! There are children reading here, buddy.