Tigra, our tortoiseshell cat with enough sass to run a small kingdom, had just turned one when she met Quigley. Her first year was spent ruling over Sammy, our sweet 6-pound brown and tan Yorkie, who never minded being bossed around. They were an odd but inseparable pair—until Sammy passed away. The house felt quieter after that, too quiet. Then Quigley came crashing in like a wind-up toy with no off switch. Tigra wasn’t sure what to make of him at first, but it didn’t take long before she...