Cats are stereotypically stealthy, quick, graceful, and acrobatic. Ninja are stereotypically stealthy, quick, graceful, and acrobatic. So of course, in a work involving both cats (whether as literal Talking Animals, merely symbolic Animal Motifs, or something in between) and ninja themes, you should expect the ninja role to tend towards cats. Cats in general have a reputation for being an adorable pet, as well as for being self-reliant, hunting down mice with either ruthless efficiency or sadist