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, as well as a few other white spots.
His first appearance is here, where he describes Ellis as his "favorite littermate." This implies that flying-cat familiars are born, not made (which isn't totally correct), and also makes you wonder how much of a pain in the ass the other members of that "litter" must be. In reality (sic), they're not exactly littermates, as Jim Hanson, creator of the Fun with Familiars series, explains here, but the term will do for now.