To continue my previous thought... Something like this?
Boggart - my pet cat (Item of Power, -1)
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One-Time Discount (+2) - it's a cat. It's just as noticeable as a cat is.
Claws (0*) - it's a cat. It's got claws. They may not be big, scary claws, but they're claws.
Echoes of the Beast (0*) - ...it's a cat. "echoes" indeed.
Diminutive Size (0*) - Boggart is not a big cat. He weighs all of like 5 pounds.
Marked by Power (0*) - Boggart is a cat... But to anyone who's sensitive enough, he's blatantly not *just* a cat.
Supernatural Sense (-1) - it's not only a cat, it's a cat infused with my magical powers. He's supernaturally aware. In this case, probably vision.
Refinement (-2) - when Boggart is with me, I gain +1 control and power to each of air and spirit magic. He's a creature of air and shadow to start with, and now he's infused with my power as well. He "smoothes the flows", so to speak.
In the above, I took Claws, Echoes of the Beast, Diminutive Size, and Marked By Power as free under the "It Is What It Is" clause of Items of Power. It's a cat, with all the advantages and disadvantages that implies - he's cute, adorable, small, agile, and cunning, but also weak and vulnerable. And Items of Power specifically must be "obvious as something unusual" - which seems like exactly the definition of Marked by Power, in my reading. He's a cat, but anyone with even the tiniest bit of sensitivity knows he's not a normal cat.
As a GM, I'd accept the "free" four points of powers, because while they're advantageous they're also counter-balancing the strong disadvantages Boggart has for a PC over, say, Amorrachius. The sword, for example, is neither soft and squishy nor prone to disagree with your relative priorities on the problems in your life.
EDIT: Oh, and as he's a cat - and probably smarter than the average domesticated (*cough*) animal - he should have an aspect or two reflecting it. I'd vote for "Superior Stare" and "*yawn* Don't Interrupt My Beauty Sleep With Your Pitiful Human Problems". :-P