The 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 Casey's lighthearted nature and tendency to joke around, as indicated in the character description. Her grin while listing the reasons suggests her playful attitude, even when discussing a potentially serious issue like a mouse in the house.