The piece of dialogue that would best help develop Casey's character for readers is:
“Lots of reasons. There were nibbled food boxes, the scratching sounds in the wall—oh!” Casey grinned. “And I saw a mouse in the kitchen this morning.”
This line showcases her lighthearted nature through her playful grin and the way she lists out the reasons why she believes there's a mouse, blending her humorous side with the seriousness of the situation. It balances her tendency to joke around while showing her confidence in what she’s saying.