Symbolism, Imagery, Allegory
Despereaux's way too small for a regular suit of armor. He'd get lost inside a soup can. But one night, Despereaux has a terrible dream about a suit of shining armor that turns out to be empty. He's horrified by this dream because it means that there's no knight inside; no one's coming to save him from his terrible fate. The dream means to him that there are no happy endings, no rescues. But later on, when Despereaux finds out that Mig and Roscuro have kidnapped Princess Pea, he thinks about the dream differently. It might be a sign that the suit of shining armor needs to be filled… by Despereaux:
"I'll have to do it myself," said the mouse. "I will be the knight in shining armor. There is no other way. It has to be me." (41.31)
We all have a suit of armor, the storyteller seems to be saying—an inner courage that we just have to step into for it to work. It's only empty when we choose not to act.