Kiki

Kiki is a little black cat who is almost always with Dark Side Cat.

In one episode, Kiki reveals the Dark Side Cat's past, which he had told her. Until Kiki revealed it, no one else had known of Dark Side Cat's past, not even the other cats. Because Kiki is the only character we know the Dark Side Cat has entrusted his past to (or spoken to at all), it's clear that he is fond of her. Or can at least put up with her.