The piece of dialogue that would best help develop Casey's lighthearted character 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 response showcases her playful nature with the use of a grin and the phrase "oh!" which adds a lighthearted tone, even while discussing a potentially serious issue. It emphasizes her tendency to find joy and humor in situations, aligning with the character description of Casey being someone who likes to joke around.