Lucy is the youngest of the Pevensie children. Not that it stopped her from becoming known as Lucy the Valiant during her time as Queen of Narnia. Pretty nice title for such a young lady.
In Prince Caspian, Lucy doesn't play as large a role plot-wise as she did in The Lion, the Witch, and the Wardrobe. Think about it: it's Caspian who starts the rebellion and Peter who leads the army to victory. Edmund plans their route to Caspian's camp, and Susan, um, complains. Lucy? Well, she sees Aslan, tells everyone, and…that's that.
But just because Lucy doesn't do much, it doesn't mean she can't hold her own. Thematically, this girl's got a lot going on.
As the one who discovered Narnia first, Lucy has a special connection to the magical land; and of all the children, she's the one who never stops believing in it—or in Aslan. That means she plays the exact same role in Prince Caspian that she did in The Lion, the Witch, and the Wardrobe: she's the character of faith.
Her time to shine in this particular story comes when the Pevensies and Trumpkin are trying to find their way to Caspian's camp. As you'd expect from any good narrative, they get lost and come to a literal (and proverbial) fork in the road. That's when Lucy sees Aslan. She knows he wants them to follow, and when the others question whether or not Lucy actually saw Aslan, she pleads "'I didn't think I saw him. I saw him'" (9.63).
Clearly, when it comes to Aslan, Lucy has no doubts; her faith is absolute. But she doesn't succeed in convincing the others, and they head in the wrong direction for the better part of a day. We don't know if you've ever walked the wrong way for an entire day, but, yeah, no fun.
Still Lucy gets another chance to convince the others she was right. When she finally reunites with Aslan, they talk about what Lucy has to do:
"Will the others see you too?" asked Lucy.
"Certainly not at first," said Aslan. "Later on, it depends."
"But they won't believe me!" said Lucy.
"It doesn't matter," said Aslan. (10.63-66)
This conversation is the key to the novel's take on both Lucy as a character and "Religion" as a theme. What the others believe or don't believe doesn't matter. It's what Lucy believes and what she knows is right that matters, and she must act accordingly. Lucky for her, the others do eventually get around to seeing Aslan, so you know, all's well that ends well.
Still, we wonder if the other Pevensies ever get tired of Lucy being right all the time. Our guess is that they must secretly love it. Why? Because they let it happen—All. The. Time.
Seriously, guys, take a hint.