The piece of dialogue that best helps develop Casey’s character as lighthearted and humorous while still addressing a serious concern 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 shows Casey's playful nature with the grin and the upbeat tone, even in a situation where she's discussing a mouse in the house. It captures her lightheartedness while also presenting her evidence for the mouse’s presence.