Locke

From The Heretic Knowledge Vault

Locke hasn't appeared in the actual story yet, but since he's in the ongoing "Fun with Familiars" filler art, he gets an article of his own. He's another familiar designed along the same lines as Ellis, i.e., a winged, talking cat. He's distinguishable from Ellis by a white patch on his chest and another at the end of his tail.

His first appearance is here, where he describes Ellis as his "favorite littermate." This implies that flying-cat familiars are born, not made, and also makes you wonder how much of a pain in the ass the other members of that litter must be.