The cat is a demon.
Not black as night with glowing yellow eyes, true, but if a stub for a tail and heterochromatic eyes don't scream "demon" then Petra doesn't know what does. And screw ironically naming a predominantly-white calico cat "spotted" in a proto-language; that's just pretentious and everyone knows Kerberos is a hellhound. The cat's coat is also a van pattern, making him look much more like he has three heads than spots.
It's not pretentious and his elegant feline companion is nothing like a scruffy canine, Levi hisses, because "everyone" knows Cerberus is the guardian of the underworld. Kerberos is well-behaved and quiet, and rodents aren't a problem.
Petra doesn't dispute that the cat is well-behaved, but the one time she slept over he crawled up on her chest like some kind of incubus. She's not buying Levi's assertions that he's friendly because it's creepy how alertly he watches her and having a soft voice isn't the same thing as being quiet—has Levi heard the unnatural range of sounds that cat can make?
During their honeymoon in Paris Levi complains that pigeons are the real demons: they're pestiferous, their excrement is everywhere, and their beady red eyes just scream pure evil. Petra tells him he shouldn't eat his breakfast croissant on their hotel room balcony if he doesn't want to be mobbed.
She lets Levi keep Kerberos and warms to him though. She just insists they close the bedroom door at night.
A/N: Kerberos is a Japanese Bobtail.