One of the smallest agathions, and one of the most animal-like, the silvanshee can pass almost unnoticed in the worlds of men and cats alike. This makes silvanshees stealthy spies for the forces of Nirvana, excellent guardians of the wilderness, and capable familiars for adventurers.
The Bestiary 2 notes that silvanshees are also called cat sìth or cath sidhe…but traditionally these creatures have a much more dubious reputation in folklore, being faeries, ghosts, or transformed witches. In your campaign maybe this is just a case of mistaken identities—agathions getting blamed for the crimes of evil witches’ familiars, for instance—or maybe cat sìth/cath sidhe are a separate species entirely. Swap out the good-aligned elements from the silvanshee stat block for more fey or evil traits (like vulnerability to cold iron or the evil subtype) and you have a perfectly serviceable antagonist ready to claw, taunt, and torment the party, only to vanish the instant before it receives its comeuppance.
When adventurers come into possession of a mystical orb, they also gain a new companion—a magical talking cat. The cat aids them in all endeavors that aren’t outright evil (up to and including theft—it finds mortal notions of property amusing). It also refuses to touch the orb but seems inordinately concerned with the well-being of the artifact’s owner, inquiring often about his or her mental state and if he or she has any experienced unusual dreams of late.
Archibald Dawnfoot is a swashbuckling ratfolk eldritch knight with an unusual familiar: a rather querulous silvanshee. The cat-like agathion spends much of his time complaining about the indignity of having to accompany Archibald on his adventures, and constantly threatens to eat Archibald “like the rodent you are, no matter how overgrown.” Of course, anyone who has been on the receiving end of one of the silvanshee’s heroic strength-enhanced pounces knows how loyal the agathion really is to his master.
An eldritch lightning strike split a guardian silvanshee from his shadow. The weakened silvanshee Moss Scratcher guards his forest the best he can, but his evil twin, the genderless Shadowgrim, works to oppose him at ever turn. Moss Scratcher goes out of his way to befriend adventurers who might be able to aid him, provided Shadowgrim hasn't already turned them against anything feline.
—Pathfinder Bestiary 2 21
Great role-playing models for less benign silvanshees include Mogget from Garth Nix’s Old Kingdom trilogy or, if you want to go downright evil, Cat Sith, the malk from The Dresden Files by Jim Butcher.
Edit: Apologies for the tardiness of this entry, especially to Rory and the other readers who have written in asking for it. Original post: Hey all! I got a last-minute opportunity to go to the beach, so the silvanshee entry is going to be a tad late. Thanks for your patience—I promise this, the shinigami, and the completed sepid post will all be up ASAP.