The piece of dialogue that would best help develop Casey's character for readers is:
“I know I like to joke around a lot, but I’m not joking this time,” Casey said. “I saw the mouse myself, and it’s been eating things and running around in the walls.”
This line shows Casey's lighthearted nature and her tendency to joke, but it also emphasizes her seriousness this time, illustrating her determination to communicate the situation to her mother. It captures her personality effectively by balancing her usual jovial demeanor with the urgency of the current scenario.