From the southern forests, there was rumors of the hedge witch who had her familiar, a cat she summoned from the stars in a dark pact. That witch died long ago, but other hedge witches sometimes travel to her former cottage to try and re-enact the rite. In fact, a few hermetic Magi have performed similar rites for power.
Visier is a beautiful female feline, with the sleekest black fur, that flows in the shadows like spilled ink. Her ears are large, and her eyes look like twin slivers of the moon. Visier has served many witches, and gained more knowledge and power with each iteration, even if she loses most of the knowledge when she is 'killed' by pact-breaking. Visier grants her patron, when they swear oaths together, wisdom and knowledge in their magical field in return for being given feline rulership of their domain - in this case, no other cats are allowed in her new demesne, and the humans must keep her fed and fat. Oddly, Visier has no aversion to Magical (or Infernal!) cats, but does hate other Faerie cats.
I am aware that my numbers are way, way off. I didn't have time to balance this as a player character. But this isn't designed to be...balanced.
Visier will refuse to form an Oath with someone who has a familiar. To take Visier as a familiar, a magus would have to effectively pretend that Visier was a real familiar. Visier will also become more and more demanding as time passes on, though most of her demands will be inflicted upon the covenfolk rather than the magus. If you're using the Covenants lab customization rules, Visier counts as a Greater Guardian and Faerie Ingredients, as she brings home fun things she found prowling.
At some point, the magus might want to be rid of the troublesome faerie, perhaps because the covenfolk are becoming worn out of her demands, or perhaps so the magus can get a true familiar. Breaking this oath is quite mentally straining - though the flaws can be surprisingly chased off by keeping a number of cats in your laboratory. The oathbreaking can actually be dispelled as a level 50 Mentem effect.
Extra plot seed: Perhaps the magi hear the story of a a magus who is looking to pass his familiar off to a successor (which is normally unheard of, but the Merinita is trying to get a real familiar, and has found a loop-hole to get himself out of the oath!)
Extra Extra plot seed: Visier would actually be plausible familiar for a Bjornaer.
Faerie Might: 20Characteristics: Int +2, Per +2, Pre +1, Com -1, Str -7, Sta -1, Dex +2, Qik +3Size: -3Virtues and Flaws: Faerie Beast; Faerie Sight, Personal Powers, Positive Folktales, Pretentious, Ritual Powers x4; Envious (major), Little, Obsessed (lab research), Might Recovery Requires Vitality (fed by human hands), Succeptible to Deprivation, Traditional Ward(mortal cats)Personality Traits: Envious +6, Prideful +3, Curious +3Reputations: A skilled mystical familiar 3 (among folk witches)Pretenses: Athletics(flight) 3, Awareness(cats) 5, Brawl (claws) 4, Charm(for food) 4, Folk Ken(children) 3, Guile(innocence) 3, Language Local 4, Language Latin 4, Org Lore: Folk Witches(personalities) 3, Org Lore:Order of Hermes (personalities) 1Powers:Familiar's Oath (20 Might): Visier can swear an oath to serve a magus or hedge witch as a familiar. While they are bound together, the patron must take care of Visier, as well as give her satisfying research. This grants the patron (if a Hermetic Magus) the virtues of Inventive Genius and Great Intelligence for the duration of the familiar bond. (You could also have Visier offer her patron Puissant Magic Theory, or Free Study, or some other lab-bonuses if preferred). Should the magus break the terms of the Familiar Oath, the gifts will be withdrawn as Visier 'dies' from the betrayal. This soul-crusing experience will instead replace the virtues with Creative Block and Succeptibility to Faerie Power, or some other appropriate flaw. (Ritual Powers x4 - two for the oath, and two for the granted virtues)Flight(2 might): Visier can walk on air, little puffs of moonlight coming out from under her paws as she does so.Vis: 4 Vim vis in eyes