The best piece of dialogue to 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 dialogue showcases Casey's lightheartedness with her grin and playful tone, while also presenting her concern about the mouse. It balances both her jokey nature and her serious claim, emphasizing her character in a way that aligns with the description provided.